My favourites are:
Flashback (All formats) - Realtime rendering on a Megadrive... Delphine software at their best.
Unreal - Remember it being pretty damn smart, a bit like a demo from the demo scene!
Street Fighter II (SNES) - Blew my mind away at the time with the sheer amount of different endings.

My worst ones are:
Manic Miner 2049er (Atari 800XL)- ...because there wasn't one, Atari you turds!
Half-Life - Nice music in the credits, but thats about it, ace game though (Goes for alot of FPS's aswell!