I like both. But there are only few multiplayer games that I've enjoyed.
Some (online) multiplayer games that I've enjoyed a lot: UT99, CS 1.3-1.6, FLF (a HL mod), WC2, WC3, Diablo 2, StarCraft, WoW.
SSBB, Tekken 2 and couple of FIFA's would be some offline multiplayer example (SSBB online as well though).

Single player. My favorite genres are TBS and RPG. I hardly ever play multiplayer, be it online or offline. The last one may change with Modnation Racers however.

And I absolutely hate MMORPGs.

If was between multiplayer only (like MAG) and single player only (like AC2) I'd chose single player, but when the game gets cheaper.

Depends on the game.

A racer would generally lead me to say multiplayer, although there are exceptions for that too, my most recent purchase being F1 2009 for Wii, which has no online component... and I am glad of it, because I can be pretty sure playing against real people would be a horrible experience for me playing Formula 1.
If Nintendo make an F-Zero for the Wii though, and don't include some sort of online then I will cry (30 player online would be awesome, but I would be satisfied if they can't manage that many and take it down to 12 player like MKWii)

Obviously multiplayer adventure games rarely work as well as their single player versions, while FPS are much more a multiplayer thing, often with a lacklustre single player tacked on.

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Overall I will say single player, and my personal game time more than backs this up (I probably spent 90%+ of my gaming time on single player aspects of games) but I still love multiplayer.

depends on my mood sometimes single player sometimes multiplayer but when i wanna play single player i can't really find alot of games with a great single player.

This is VGC, of course single player wins.

Personally I voted multiplayer, even though Oblivion is my favorite game all time.

Age of Empires, MMORPGs, online shooters. Multiplayer rocks!

Single Player. Other than Borderlands online co-op recently I cant remember the last time I played a multiplayer game.

3 of my most anticipated titles next year are all single player, God of War 3, Heavy Rain and Fallout: New Vegas

I never play online multiplayer (find it a bit boring), Local Multiplayer is extremely enjoyable with right games like NSMB or Mario Kart.

Most of games that i enjoy are Single Player Titles

I voted Single Player, i prefer games with a good and unforgettable story like MGS, FF or simply amazing single player experiences like Bioshock, Valkyria Chronicles, etc.

Sometimes i like to play multiplayer like the one from CODMW2 or Motorstorm 2, but it doesn't fascinate me like epic single player experiences.

Since I mostly play RPGs, single player.
After that comes offline multiplayer, I have enjoyed some shared gaming with Tales of games, as well as with some games obviously geared towards it (Guilty Gear, BlazBlue.. and pretty much any fighting game, but those are the two I like best).

I don't really mind online multiplayer, I did quite a bit of MMOing until it started affecting my grades and it was fun - but it takes time to get to know the right people and anyone random comes with the potential risk of being a huge annoyance.