All time: Paradroid - C64
This game took so many of my nights, even when I'd complete it I'd start it all over again and again. I wish it would be released on the North American VC soon, if not then I'm looking for a C64DTV.
Wii: Metroid Prime 3 Corruption
The first Wii game that drew me in and wouldn't let go. Played through it 3 times for a total of just over 60 hours, which is pretty good for a 16 hour game.
WiiWare: World of Goo
Wonderfully different and a ton of gooey fun.