Mario Kart Wii is my current go to game.

GoldenEye is my all time go to game.

Any of the Castlevania games that don't hold your hand, all Mega Man games except 8, occasionally Brawl and F-Zero GX (and I don't even like racers). Spelunky is also one.

Temco Super Bowl on NES and Mario Kart on DS, never get bored of those two games.  

Mine is Banjo-Kazooie :)

i'd say starcraft, civilization or diablo would've done the trick every time. Now, I'm just going to wait for the next installment in these series ^^

Recently, as in the last few years, I'd have to say Gears of War 2 and Starcraft as the two big ones. When I'm with friends or family and we want to play a game, it's usually Smash Bros, Goldeneye, or Mario Kart 64.

MW2.

sonic 2

Tetris or Streetfighter Alpha. Whenever I'm really bored, out with a boring person, or at a doctor's office type setting, I break out those games. Instant happiness for 30mins-1 hour!

Super Street Fighter 2 HD Remix is my online "Go to game".

Any Super Mario Bros game, but probably the original comes first. Like RubangB I speed run it all the time and occasionally take detours or do a complete playthrough if I want to get a higher score.