Here's the good stuff. Hot damn, do I love this entire package. Far and away my favorite game(s) in the series when I first played them back in '05, and the only games that have since superceded them have been newer releases. Sonic's one of those series I've been into since before I owned game consoles - I have to thank demo kiosks at stores for Sonic Adventure 2: Battle and Sonic Heroes for that - but I was never sure about 2D Sonic going into the GameCube Mega Collection. Sonic 1 and Sonic 2 lost me after a bit, but from the moment I tried Sonic 3, the game had me.
...of course, a big part of that was that Sonic 3 is the first game in the series to have a proper save feature, so when I was inevitably bad at the video game, it was easy to pick up the pieces and try again. I actually had a slightly harder time getting into the Sonic & Knuckles half for precisely that reason; yeah, it's more Sonic 3 goodness, but no save meant I had to start over at dumb ol' Mushroom Hill when I doofed up.
Save feature or no, though - both halves of Sonic 3 & Knuckles are great games in their own right. If Sonic 2 represented a start of understanding what made Sonic compelling in 2D, Sonic 3 & Knuckles represented full confidence in this knowledge. Levels are huge and expansive, naturally integrating spectacle set pieces with exploration and gameplay mechanics. Of course the most obvious thing is how each Zone shifts between the first and second Acts: Angel Island catches fire, the power goes out in Carnival Night, the magma cools in Lava Reef, etc, all complete with melodic progressions as the composition shifts from the first to the second Act's version of the music. But there are things baked into the levels, too, like the slopes of Marble Garden, the jets and hydroplaning in Hydrocity, the turns and outdoor segments of Flying Battery, the ghost chase in Sandopolis, etc... So many things that naturally invite the player to explore and experiment with the game's physics systems, so many things smartly built into the makeup of the worlds. I'd argue that the mobius strips and loop-de-loops that characterize key moments of Sonic 1 and 2 felt like they were shoehorned into the levels (I'll be honest, the loops in Green Hill look unnatural to me), while everything in S3&K feels like something that might incidentally exist within the parameters of this world, gleefully being subverted by Sonic & friends. I really do love just about every set piece here.
...and, yes, I include in that the infamous drums in Carnival Night. I don't think I had issues figuring those out? What, kids in the early 90s never thought to pump the D-Pad up and down on their own?
Another show of confidence is the expansion of playable characters. Tails now has a fully expanded moveset, with his flight being naturally integrated into level design (and something that translates into swimming! The animation of him paddling is adorable). Knuckles becomes playable with Sonic & Knuckles, presenting a slower but more exploration-oriented approach to levels. Not to be outdone, Sonic gets access to the Insta-Shield (which I never really experimented with, admittedly), plus those nifty elemental shields' secondary functions. Having a full 4 different ways to play the game adds a ton of replayability to the experience, particularly since the routes themselves through the game change bit by bit. Tails is good for cutting your teeth on the game (though good luck with that Marble Garden boss), Sonic solo/Sonic & Tails represent a standard run, and Knux is sort of a bonus round.
And those bonus stages, though! Not content with the standard 1 Special Stage archetype, the team ultimately wedged FOUR of them suckers in here! Even my least-favorite of these, the glowy sphere one, is a solid enough time. But, like, Blue Sphere? Hoh baby, I love love love Blue Sphere. Finally, a bonus stage that represents a completely different challenge from the usual cadence of 2D Sonic gameplay but derives its difficulty from the same sort of speed and careful planning that high-level Sonic play asks for. I love that feeling of slowly-building panic I get as the game speed picks up, and I try to track down whichever Blue Spheres I've missed. It's a completely fair game, one that has lots of regular means of optimization - not just route-planning overall, but also that question of how best to turn each grid of Blue Spheres into Rings. And the music builds and builds and grows faster and faster with each loop, and... man! I love love love it. And I love that there are over ONE HUNDRED MILLION different variations! Yeah, I know they're semi-randomly generated, but still!
It's genuinely hard for me to find things to complain about with Sonic 3 & Knuckles. If I had to complain, I guess I don't love Mushroom Hill? It makes sense if you're playing Sonic & Knuckles alone, but in the context of a combined game, it feels like a second Green Hill wedged into the game's midpoint. There's also something to Sonic & Knuckles that makes it feel way shorter than Sonic 3, even though the two halves have roughly the same amount of stages. Maybe it's that the Special Rings go away after Lava Reef, so playing after that without all Chaos/Super Emeralds feels like an extended failed run? Um... I sorta think Sonic 3 ends on a weird place, like it's pretty obvious that Launch Base is a fake final level in retrospect, with Big Arm cobbled together as something that can approximate a final boss...
Oh, actually, speaking of Launch Base - I think it's a bit mean to have the timer running for that extended sequence where Sonic and/or Tails rides an Egg Mobile over to the bottom of the Death Egg. I got screwed over in an early run at that point, clearing the Ball Shooter with like 9:30 on the clock. I had to slooooooowly run out the rest of my lives with Time Over after Time Over, trying fruitlessly to see the cutscene through. In retrospect it's a minor thing, but it was pretty annoying at the time!
Most of the time, when Sonic does throwbacks to its Genesis roots, it does so specifically to Sonics 1 and 2. There are a number of reasons for this - the first entry tends to win out with these sorts of throwbacks, Sonic Team was more heavily involved with those games, Spin Dash was introduced in Sonic 2, Sonic 3 proved to be less of an evolutionary throughline for the series, Developer SEGA Technical Institute was kind of an internal punching bag, etc etc etc. I get it, but there's a part of me that qualifies how good a Sonic game is based entirely around if its throwbacks harken to Sonic 2 or 3. Sonic Heroes, Sonic Rush, Sonic 4? Bah! Sonic Mania? Now THAT'S where it's at. This is reductivist, of course, but I mention it mostly to represent how GOOD I think S3&K is. If Donkey Kong Country is a series that unquestionably hit its stride in 2 and spent its third title with lower-stakes experiments, Sonic the Hedgehog is a series that hit its stride in 2, then redoubled its pace and really hit its stride in 3 & Knuckles.
...of course, a big part of that was that Sonic 3 is the first game in the series to have a proper save feature, so when I was inevitably bad at the video game, it was easy to pick up the pieces and try again. I actually had a slightly harder time getting into the Sonic & Knuckles half for precisely that reason; yeah, it's more Sonic 3 goodness, but no save meant I had to start over at dumb ol' Mushroom Hill when I doofed up.
Save feature or no, though - both halves of Sonic 3 & Knuckles are great games in their own right. If Sonic 2 represented a start of understanding what made Sonic compelling in 2D, Sonic 3 & Knuckles represented full confidence in this knowledge. Levels are huge and expansive, naturally integrating spectacle set pieces with exploration and gameplay mechanics. Of course the most obvious thing is how each Zone shifts between the first and second Acts: Angel Island catches fire, the power goes out in Carnival Night, the magma cools in Lava Reef, etc, all complete with melodic progressions as the composition shifts from the first to the second Act's version of the music. But there are things baked into the levels, too, like the slopes of Marble Garden, the jets and hydroplaning in Hydrocity, the turns and outdoor segments of Flying Battery, the ghost chase in Sandopolis, etc... So many things that naturally invite the player to explore and experiment with the game's physics systems, so many things smartly built into the makeup of the worlds. I'd argue that the mobius strips and loop-de-loops that characterize key moments of Sonic 1 and 2 felt like they were shoehorned into the levels (I'll be honest, the loops in Green Hill look unnatural to me), while everything in S3&K feels like something that might incidentally exist within the parameters of this world, gleefully being subverted by Sonic & friends. I really do love just about every set piece here.
...and, yes, I include in that the infamous drums in Carnival Night. I don't think I had issues figuring those out? What, kids in the early 90s never thought to pump the D-Pad up and down on their own?
Another show of confidence is the expansion of playable characters. Tails now has a fully expanded moveset, with his flight being naturally integrated into level design (and something that translates into swimming! The animation of him paddling is adorable). Knuckles becomes playable with Sonic & Knuckles, presenting a slower but more exploration-oriented approach to levels. Not to be outdone, Sonic gets access to the Insta-Shield (which I never really experimented with, admittedly), plus those nifty elemental shields' secondary functions. Having a full 4 different ways to play the game adds a ton of replayability to the experience, particularly since the routes themselves through the game change bit by bit. Tails is good for cutting your teeth on the game (though good luck with that Marble Garden boss), Sonic solo/Sonic & Tails represent a standard run, and Knux is sort of a bonus round.
And those bonus stages, though! Not content with the standard 1 Special Stage archetype, the team ultimately wedged FOUR of them suckers in here! Even my least-favorite of these, the glowy sphere one, is a solid enough time. But, like, Blue Sphere? Hoh baby, I love love love Blue Sphere. Finally, a bonus stage that represents a completely different challenge from the usual cadence of 2D Sonic gameplay but derives its difficulty from the same sort of speed and careful planning that high-level Sonic play asks for. I love that feeling of slowly-building panic I get as the game speed picks up, and I try to track down whichever Blue Spheres I've missed. It's a completely fair game, one that has lots of regular means of optimization - not just route-planning overall, but also that question of how best to turn each grid of Blue Spheres into Rings. And the music builds and builds and grows faster and faster with each loop, and... man! I love love love it. And I love that there are over ONE HUNDRED MILLION different variations! Yeah, I know they're semi-randomly generated, but still!
It's genuinely hard for me to find things to complain about with Sonic 3 & Knuckles. If I had to complain, I guess I don't love Mushroom Hill? It makes sense if you're playing Sonic & Knuckles alone, but in the context of a combined game, it feels like a second Green Hill wedged into the game's midpoint. There's also something to Sonic & Knuckles that makes it feel way shorter than Sonic 3, even though the two halves have roughly the same amount of stages. Maybe it's that the Special Rings go away after Lava Reef, so playing after that without all Chaos/Super Emeralds feels like an extended failed run? Um... I sorta think Sonic 3 ends on a weird place, like it's pretty obvious that Launch Base is a fake final level in retrospect, with Big Arm cobbled together as something that can approximate a final boss...
Oh, actually, speaking of Launch Base - I think it's a bit mean to have the timer running for that extended sequence where Sonic and/or Tails rides an Egg Mobile over to the bottom of the Death Egg. I got screwed over in an early run at that point, clearing the Ball Shooter with like 9:30 on the clock. I had to slooooooowly run out the rest of my lives with Time Over after Time Over, trying fruitlessly to see the cutscene through. In retrospect it's a minor thing, but it was pretty annoying at the time!
Most of the time, when Sonic does throwbacks to its Genesis roots, it does so specifically to Sonics 1 and 2. There are a number of reasons for this - the first entry tends to win out with these sorts of throwbacks, Sonic Team was more heavily involved with those games, Spin Dash was introduced in Sonic 2, Sonic 3 proved to be less of an evolutionary throughline for the series, Developer SEGA Technical Institute was kind of an internal punching bag, etc etc etc. I get it, but there's a part of me that qualifies how good a Sonic game is based entirely around if its throwbacks harken to Sonic 2 or 3. Sonic Heroes, Sonic Rush, Sonic 4? Bah! Sonic Mania? Now THAT'S where it's at. This is reductivist, of course, but I mention it mostly to represent how GOOD I think S3&K is. If Donkey Kong Country is a series that unquestionably hit its stride in 2 and spent its third title with lower-stakes experiments, Sonic the Hedgehog is a series that hit its stride in 2, then redoubled its pace and really hit its stride in 3 & Knuckles.