My new favorite is Ristar for Sega Genesis. I played it when I was young and just recently realized it was on VC. If you don't own it I recommend trying it out. This is basically a game that resembles Sonic play except for the fact that you can't just hold the right arrow, hitting A occasionally, to win a level. You have to take it slow or you'll be killed. (You have life in Ristar, unlike Sonic.) To end this long summary, I have to say that the water levels are great to play on. You don't have to worry about air because it's unnecessary in this game. Also you're able to swim instead of having to jump. So basically, if you enjoyed Sonic games on the Sega, and even if you didn't, you'll enjoy Ristar. So:
N64: Mario Kart 64. Can't beat the original (at least until the Wii one comes out).
NES: Donkey Kong (lol)