When I played this game for the first time in 2017, I declared it my favorite Sonic game. These days, I do ultimately have to give the edge to Generations, since it does feel like a more unique experience overall, but Mania still manages an easy second place and is definitely the best 2D Sonic game in my opinion.
This obviously wasn't the first attempt by Sega or Sonic Team to recreate the classic formula, but while I do like, say, Classic Sonic in Generations and the Advance games, I think this is the only game to really "nail" the feeling of the classic games. The controls basically feel 1:1 with the classic games, although does come with the addition of the 'Drop Dash' ability - which admittedly has never really been something I've taken much advantage of in this game (I didn't even really know it was a thing until about a year or two later lol), but it is a nice extra control ability that does add a bit more depth in speedrunning.
The level design is where the game stands out the most for me though. While I did like what Sonic 3 & Knuckles was going for by mixing elements of exploration, speed and platforming, I do feel like the game did break the pace quite a bit. That obviously isn't to say I dislike the game (it's still among my Top 5), but I do think this game improves upon that balance by not breaking the pace quite as much. There are definitely parts of the game that slow down, but for the most part it still manages to keep up a consistent fast pace throughout akin to Sonic 2 while keeping the scope of Sonic 3 & Knuckles' level design - basically combining the best elements of the classic games in general. Levels are full of little nooks and crannies with extra items to look out for, including the Special Stage rings, as well as alternate pathways that makes Mania open up in terms of how one plays the game - whether that be through speedrunning, replaying the levels, exploring the levels, or whatever. A lot of people do criticize the overabundance of reused 'classic' level themes (basically being 2/3rd of the game's levels), and I do understand that, but at the same time this game still manages to make even those levels feel unique and new with how they're designed.
Presentation is also really good. It really feels like they went for the 'Saturn' style and quite frankly, Sonic Mania has always felt like a lost 2D Saturn Sonic game (if they made that instead of trying to jump on the 3D train). The sprites of the main characters are enhanced with more animations, color, and resolution while still staying true to the Sonic 2 style, and the game mixes that occasionally with more Saturn-like assets, especially when it comes to the Special Stages. The soundtrack is also really good, and I don't think I need to go into much detail about that.
My only real major criticism of the game is that the bosses are... a mixed bag. Which is the case for most Sonic games to be fair, but this game can definitely go from having really good bosses and concepts (i.e. the Egg-Robo being the Green Hill Zone boss, the Mean Bean boss in Chemical Plant), but especially later in the game, it feels like there's a lot of "wait for thing to happen" kind of bosses (i.e. Flying Battery Act 1) as well as some parts of bosses that just feel ripped straight from the classic games (i.e. the last part of the Hydrocity Act 2 boss). I also feel like Flying Battery Act 2's boss has a somewhat weird hitbox.
Other than that, Sonic Mania really manages to make for a great celebration of the series, taking what worked well about the classic games, staying true to that formula, but building upon it and making it feel like new. Plus, even now, it does bring back a lot of memories of playing the classic games on my DS back in the day, which definitely gets some bonus points for that alone.
This obviously wasn't the first attempt by Sega or Sonic Team to recreate the classic formula, but while I do like, say, Classic Sonic in Generations and the Advance games, I think this is the only game to really "nail" the feeling of the classic games. The controls basically feel 1:1 with the classic games, although does come with the addition of the 'Drop Dash' ability - which admittedly has never really been something I've taken much advantage of in this game (I didn't even really know it was a thing until about a year or two later lol), but it is a nice extra control ability that does add a bit more depth in speedrunning.
The level design is where the game stands out the most for me though. While I did like what Sonic 3 & Knuckles was going for by mixing elements of exploration, speed and platforming, I do feel like the game did break the pace quite a bit. That obviously isn't to say I dislike the game (it's still among my Top 5), but I do think this game improves upon that balance by not breaking the pace quite as much. There are definitely parts of the game that slow down, but for the most part it still manages to keep up a consistent fast pace throughout akin to Sonic 2 while keeping the scope of Sonic 3 & Knuckles' level design - basically combining the best elements of the classic games in general. Levels are full of little nooks and crannies with extra items to look out for, including the Special Stage rings, as well as alternate pathways that makes Mania open up in terms of how one plays the game - whether that be through speedrunning, replaying the levels, exploring the levels, or whatever. A lot of people do criticize the overabundance of reused 'classic' level themes (basically being 2/3rd of the game's levels), and I do understand that, but at the same time this game still manages to make even those levels feel unique and new with how they're designed.
Presentation is also really good. It really feels like they went for the 'Saturn' style and quite frankly, Sonic Mania has always felt like a lost 2D Saturn Sonic game (if they made that instead of trying to jump on the 3D train). The sprites of the main characters are enhanced with more animations, color, and resolution while still staying true to the Sonic 2 style, and the game mixes that occasionally with more Saturn-like assets, especially when it comes to the Special Stages. The soundtrack is also really good, and I don't think I need to go into much detail about that.
My only real major criticism of the game is that the bosses are... a mixed bag. Which is the case for most Sonic games to be fair, but this game can definitely go from having really good bosses and concepts (i.e. the Egg-Robo being the Green Hill Zone boss, the Mean Bean boss in Chemical Plant), but especially later in the game, it feels like there's a lot of "wait for thing to happen" kind of bosses (i.e. Flying Battery Act 1) as well as some parts of bosses that just feel ripped straight from the classic games (i.e. the last part of the Hydrocity Act 2 boss). I also feel like Flying Battery Act 2's boss has a somewhat weird hitbox.
Other than that, Sonic Mania really manages to make for a great celebration of the series, taking what worked well about the classic games, staying true to that formula, but building upon it and making it feel like new. Plus, even now, it does bring back a lot of memories of playing the classic games on my DS back in the day, which definitely gets some bonus points for that alone.