Super Smash Bro's I reckon, just loved the mayhem and the speed of the game. It's great to play on your own and even better which friends Have some great memories playing it with my friends before heading out into town.
At the moment, maybe Majora's Mask 3D or Luigi's Mansion Dark Moon. Both excellent 3DS games which are extremely fun to play (even if unfortunate circumstances stop me from being able to play the latter any more).
Of all time? Well, it depends. For multiplayer games, Smash Bros (especially Brawl) is pretty good, especially with friends. For single player, I still have some nostalgia for Wario Land 4. Yes, it's an old school 2D platformer, but the music is fantastic, the levels are really well designed and the graphics themselves are really nice too. Especially when you consider that someone hand drew everything in this title rather than relying on a bunch of modern effects to cover it.