I'll put one for each system I've played on.
NES- See, told ya. ActRaiser. Five seconds after posting.
PS1- Some racing game. I forget what it was called, but I played it for days straight.
N64- Legend of Zelda: Ocarina of Time
Gamecube- Super Smash Bros. Melee
Wii- Guitar Hero... Not sure which one I'd say is my favorite, though. I liked Legends of Rock's soundtrack better, but World Tour has the multiple instruments, and more little features... Probably World Tour, as I find more songs I like every few days.
Xbox 360- Assassin's Creed for single player, L4D or Halo 3 for multiplayer. The multiplayer was all I really liked on Halo 3.
Gameboy Color- Pokemon Yellow (My first video game ever)
Gameboy Advanced- Golden Sun & Golden Sun: The Lost Age
Nintendo DS- The World Ends with You
Overall... It's a close toss-up between SSBM, TWEWY, and GS/TLA. But probably TWEWY. The music, even though it really wasn't my style, was great, the graphics weren't bad at all, amazing story, great gameplay, very creative concept overall... If you have a DS, and don't have this game, go buy it now. RIGHT. NOW.