I just find it amusing that the classic Sonic game with the time travel gimmick is so chronologically confusing. Due to being developed at the same time, Sonic CD was released after Sonic 2 but takes place before it, and seems a lot closer in graphical style to the original game!
Anyway, this makes a really interesting foil to Sonic 2 because it tries a lot of different ideas both for better and worse, the most obvious one being the time-travel mechanic. Being able to travel backwards and forwards in time means lots of additional content (four different variations of each stage!), and the optional goal of destroying machines in the past that will change the future for the better adds some replay value to what is actually a pretty easy base game. Through this format, Sonic CD casts itself as a time/score attack game, where each individual stage is short and relatively easy but you're encouraged to try them again and again, and rewarded with different content and a slightly different ending once you get good enough.
However, the execution is slightly off here: playing simply to get through the game is a pretty bland and frictionless experience, while trying to get the good ending requires tedious combing through levels trying to find the right machine to destroy, and there is very little middle ground between these two experiences of the game. By contrast, Sonic 2 lacks any such overarching gimmick but has much tighter level design, more interesting zones, and offers an experience with just the right amount of difficulty (except for Metropolis Zone, screw that).
Sonic CD does have one aspect that it excels in though, and that's the bosses. They're far more creative than anything in previous Sonic games, and almost none of them fall into the category of "figure out attack pattern, hit boss X number of times, profit". The two highlights for me are a pinball-themed boss (Sonic Spinball without the fluff and with better physics!), and a thematically-appropriate straight-up footrace against Metal Sonic which you simply win by getting to the end of the stage first!
I'd personally rank this slightly below Sonic 2 but it's still firmly in the "very good" category. However, the fact that Sonic CD was by far the best-received and best-selling game on the Sega CD was perhaps a warning sign that foreshadowed Sega losing more and more ground in the console wars with each successive generation. Oh, to find the right machine to break in the past to save this franchise from the bad future...
Anyway, this makes a really interesting foil to Sonic 2 because it tries a lot of different ideas both for better and worse, the most obvious one being the time-travel mechanic. Being able to travel backwards and forwards in time means lots of additional content (four different variations of each stage!), and the optional goal of destroying machines in the past that will change the future for the better adds some replay value to what is actually a pretty easy base game. Through this format, Sonic CD casts itself as a time/score attack game, where each individual stage is short and relatively easy but you're encouraged to try them again and again, and rewarded with different content and a slightly different ending once you get good enough.
However, the execution is slightly off here: playing simply to get through the game is a pretty bland and frictionless experience, while trying to get the good ending requires tedious combing through levels trying to find the right machine to destroy, and there is very little middle ground between these two experiences of the game. By contrast, Sonic 2 lacks any such overarching gimmick but has much tighter level design, more interesting zones, and offers an experience with just the right amount of difficulty (except for Metropolis Zone, screw that).
Sonic CD does have one aspect that it excels in though, and that's the bosses. They're far more creative than anything in previous Sonic games, and almost none of them fall into the category of "figure out attack pattern, hit boss X number of times, profit". The two highlights for me are a pinball-themed boss (Sonic Spinball without the fluff and with better physics!), and a thematically-appropriate straight-up footrace against Metal Sonic which you simply win by getting to the end of the stage first!
I'd personally rank this slightly below Sonic 2 but it's still firmly in the "very good" category. However, the fact that Sonic CD was by far the best-received and best-selling game on the Sega CD was perhaps a warning sign that foreshadowed Sega losing more and more ground in the console wars with each successive generation. Oh, to find the right machine to break in the past to save this franchise from the bad future...