There's been a lot of fun games, but the greatest game of all times has to be that first Nintendo Wrestling game, where you could be that spanish guy with the pink pajamas and the star on his chest who did cartwheels, or that creature-from-the-black-lagoon-guy whose special move was the Cha-Cha-Cha Face Chew.
I'm sure you could play it single-player, but I never did, and I'd feel really sorry fer you if you missed out on the sheer awesomeness of that game 'cause you didn't have any friends or a little brother to play it properly with.
See, the thing that made it great was that once you beat the shit out of your opponent, you could drag his exhausted body around the ring (and outside the ring) and torture his pitiful defenseless corpse for hours while yer buddy (or yer little brother) had to sit there screaming helplessly and begging you to finish them off and pin them so they could pick a new guy or something.
And of course you'd be yelling all that awesome macho-man wrestling shit at them the whole time, like "oooh yeeeeaah!" and "oh that's gotta hurt!" until they got fed up and tried to steal your controller away from you, and the game would suddenly erupt from the screen and become Real Life Living Room Wrestling with controllers flying everywhere and people getting rug burns.
And there was always a small chance that your opponent would get his wind back if, fer whatever reason, you weren't able to keep beating on his body with a chair or smashing his head into a turnbuckle or something, and then he'd basically come back from the dead, and he'd be able to turn the tables on you Hulk Hogan Hulkamania-Style and drag your wrestler's pitiful defenseless corpse around the screen forever and ever, smashing it on everything and howling out his own macho-man stuff in a rapture of revenge.
I just don't see how anybody could make a game better that without running the risk of somebody losing an eye.