After all the Mania hype, I still think that, concept-wise, Sonic CD is the best classic Sonic game.
What is classic Sonic? It's no average platformer, it's not just a platformer with fewer obstacles just to make you feel good rolling around at the speed of sound... (sorry, can't help it) , but it's a platformer that requires you to make good use of the environment to keep your momentum, in order to go as fast as you can. (Classic Sonic: THIS IS WHO I AM)
But in fact, some people don't seem to get the point, since classic Sonic games don't really explain those to you. You can just go throught the stage really casually, dropping rings like it's hot (sorry). I'm not saying that you can't play the game this way, since playing game IS a way of relaxing yourself after all. But the thing is that, sone people would think that Sonic is just a game that you press right to win (or boost to win, in the modern era). And thus a certain sort of Sonic haters are born.,
Now take a look at Sonic CD. Of course, you can play the game like any other Sonic game, just leave everything behind and blast through the stage with Sonic speed (sorry... too many references), but you'll run into some time travelling now and then, and over time you might realize that it's keeping the speed that helps when you wanna do a time travel. Then if you wanna travel to the past and kick some metal butts, you'll start to find places that are suitable for keeping your momentum. And then you might learn to appreciate the level designs in Sonic games. That's how I felt about Sonic CD, at least.
Also, time-traveling. I know the Sonic series involves a lot of tine-traveling in their games, and I mean A LOT. Sonic 06, Sonic generations, Sonic Mania, straight-up time traveling. Sonic Adventure and Shadow the hedehog, had the characters travel though memories of Tikal/Shadow himself, which can also be regarded as some sort of time traveling. But the thing is that, they don't exactly feel like time traveling, it just seems that they have been moved to another location with a different style of surroundings. But look at Sonic CD. Different time, same place, which can show how the exact same place changes over time. It actually feels like time traveling rather than moving to a different location. And it would show how the good 'o Doc's deeds makes everything worse (which should be an important key of how Sonic games' plots should go, since some recent Sonic games don't create the feeling that you're saving the world from Eggman's hazards, but rather like you're just walking through places, trashing some robots on the way, and beat Eggman like it's your daily job.)
And another thing is the music. I totally agree with Cobanermani456 on how the music would affect how you feel about a stage in a game. And Sonic CD's music (JP soundtrack) is just so awesome in a game of 90's, and it's still pretty good in today's standards. I love the peaceful tones of the Past, the energetic tones of the Good Future, the upbeat and serious tone of the Bad Future, and the rather generic but still good tone of the Present. All of them are really great. I once said that my fav Sonic game soundtrack is Lost World's, but I think I was wrong. CD's Japanese soundtrack should be on the top of the list without doubt.
In a word, I LOVE Sonic CD. And I really appreciate Christian Whitehead's work of remaking Sonic CD, fixing problems in tge original, and having more people adore the game.