As much as I love the Genesis Sonic games, Ristar is the king of 2D platforming.
I would have never bought Sonic Mega Collection if this game wasn't on it, even considering how Sonic 2 & 3 are some of my favorite games. Ristar is that good. It's really pretty hard to explain, but I'll try if I must. You're a star named, well, Ristar (duh). He has umm... really long stretchy arms that go in all 8 d-pad directions. He can grab enemies, poles, objects, walks, ladders. Anything that is in the game, u can pretty much grab it. This leads to really innovative and fun platforming game play. It's a really long game for not having a save feature, and pretty hard at that, but who ever complained for being given too much game and a challenge?
This is probably my 2nd favorite game in the platforming genre behind Super Mario Bros. 3, and there are not a lot of games that can hang with SMB3. This one can though. If you can't find a copy for the Genesis (which will be pretty cheap and common due to crap sales and sheer underratedness), it's an unlockable game on Sonic Mega Collection, which really would be worth the buy even without Ristar. Have fun.