Sure thing, Torgo. This ought to generate more interesting discussion, anyway.
* Sonic 3 and Knuckles - I really, really like the platforming in this game and how well each level's individual gimmicks are incorporated into that platforming. The production values are excellent, and the seperate backgrounds, musical tracks, and bosses for each individual act makes me giddy. The seperate routes for Knuckles and the seperate final bosses each character faces makes the levels and storyline more natural and believeable. And in my opinion, Sonic 3 and Knuckles incorporated its story developments into the game perfectly, without the need for cutscenes or diologue.
Now, to be fair, I'm going to list some things I don't like about S3+K. The Special Stages aren't that thrilling. Once you establish the sphere patterns and how to deal with them, they're redundant. I also don't care much for the Bonus Stages, although the Rolling Jump one is rather fun. The multiplayer mode in Sonic 2 is far superior, as I'd much rather race through full-lengthed in-game zones than little midget zones that, like the Special Stages, are too formulaic. Finally, some of the acts (like the second acts of Carnival Night and Sandopolis) are so large that you're forced to rush through them in order not to time out. This totally clashes with natural tendency and benefits of exploring the levels.
* Sonic CD - The game's aesthetics are, in my opinion, superior to even Sonic 3 and Knuckles's. The ability to travel to different time periods is a treat, and I love it how gameplay is actually altered depending on which period you go to (badniks in varying states of disrepair, the different placements of gimmicks and traps, the presence of Eggman's Machines and Metal Sonic projectors in the past). The bosses and the means to defeat them are (usually) delightfully unconventional. The Special Stages are both pretty and fun, although hunting down the time-extending UFO gets to be a drag in later rounds. Finally, the extras, especially the animations, sound test, hidden illustrations, and Wacky Workbench statues are nice.
The level design of this game, however, never feels as strong as that of S3+K, perhaps due to the rather nebulous nature of Sonic CD. In S3+K, it feels as if there are definite obstacles, and I gain satisfation for figuring out how to overcome them. In Sonic CD, I feel as if I can overcome most obstacles by simply jumping over them. The gimmicks aren't very strongly implemented into the levels (with the obvious exception of Wacky Workbench) and some, like the walker and the shrink ray in Metal Madness, just aren't very interesting. The game's spindash and rate of acceleration aren't very fine-tuned, and the latter makes Starlight Speedway are very disorienting experience. The final boss is, ironically, unintimidating and rather uncreative. Finally, the inability to select specific time periods in Time Attack mode is a bit of a turn-off.
I'll give reasons for my other choices later.