These days it's Metroid Prime Trilogy, Resident Evil 4 or Twilight Princess

last gen it was Metroid Prime, Resident Evil 4 or Wind Waker (guess not much has changed)

gen before that it was Mario Kart 64, Goldeneye or Wave Race.

i could keep going back but i can't be bothered at the moment. all the games from past generations i still go back to but not as often as the first three i mentioned.

I have League of Legends and L4D2 for some multiplayer action with my friends.

Other than that, my main single player game I keep going back to is Metroid Prime. The atmosphere in that game is amazing.

If i just need something... anything, to play, it'll be Brawl. Just randomize everything except my character, and i've got instant fun and variety to last a good bit.

When my first PS3 went bust, I pulled out the N64.

When the annoying cousins come over and I don't want them touching my new consoles, I pull out the N64.

When someone told me that Peach spoke at the end of Mario for the first time, I spent my days trying to finish the game.

When the world's just pulling me down and Killzone 2 just isn't doing it for me anymore, Super Smash Bros. comes out and friends are over.

Soooo, the N64 is my go to console ( all the games on it are fantastic so no specific one)

I'd say Rockband 2, Guitar Hero Metallica and Burnout Paradise. Altough, any kind of Sudoku or Bejeweled can do the trick

                                           This                                                   or                                              That

Tecmo Super Bowl is the shiznat.  I also find myself going back to Blades of Steel on the NES.

Recently it has been Fallout 3, Oblivion or Borderlands

All-time, Final Fantasy VIII or Mechwarrior 4

Single Player: Mass Effect
MP: Halo 3
Or if I just can't play anything else (hasn't happened since 2008) I start a new Sorceress in Diablo 2.

Tetris Columns Yoshi's Cookie etc