So, my cousin just walked up to me and asked me this. I told him I honestly don't know.

I thought this was a good question to ask, so I'm asking.

I voted singleplayer.

Though I'm biased cuz I hate people.

I don't know really. My favourite games are all Single player games, and I prefer games made for single player. But, when having friends and family over, playing Street Fighter, Mario Kart or other multiplayer games is great.

Define "multiplayer" ...online multiplayer, offline multiplayer or both?

I prefer single player over online multiplayer ...but i prefer offline multiplayer over single player

Multiplayer all the way. a game has to be REALLY good for me to want to play it alone. (I.e. Mass Effect)

Both.

for me

1) single player (depending on the kind of game, it can be cool to have someone watching)
2) offline multiplayer
3) online multiplayer

but all of these can be awesome, it just depends on the game

Singleplayer. It's seems impossible to get people into the same room anymore, and i don't care for the impersonal online-experience

Then ill have to go with multiplayer =)

I cannot honestly answer this question. It all depends on the game.
My favorite games include bot single player and Multi-player.
My favorite FPS Multi-player game is CS:S. My favorite single player FPS is the original Half-life. I play CS:S more often because of the multi-player aspect. This does not change my preference for Half-Life as a superior game.
My favorite RPG's are all single player, but will dabble in MMO's ever once in a while.
In the end, it depends on game/game type.