Single player.
I'm down with the multiplayer, but only LAN, or the convenient console alternative split-screen/system link, online multiplayer is the bane of my existence (exaggerated). It's just not my cup of tea I guess.
The main things that deter me are the sheer volume of players, with such varied skill levels that don't seem to be matched too well, kinda takes the fun out of it you know and ah.. I don't have the mental capacity right now to come up with any other reasons, but I know they're there.
Also, under no circumstance should multiplayer be the bulk of the game, oh yeah, unless it's Quake or UT or even..
(Arcade fighting games, party games, colourful band simulations, the list goes on)
Okay so it certainly has its place.
Multiplayer games are perfectly alright with me, in fact I enjoy them rather a lot, I just prefer getting together with a close group of friends and having some old fashioned gaming fun together, it just has a different feel to it. So I guess just as long as they don't cut out LAN/split-screen/offline play it's fine for multiplayer focused titles to exist (Good thing I'm not an RTS fan or I'd be pissed about Starcraft)
Overall though, I prefer single player anyway.