Single.
I'm not particularly opposed to multi-player, but Civ games take a long time (I don't like how quick games flow, even though the numbers are just raw percentages of standard, the flow seems off).
That, and I hate playing against randoms over the internet. I've done it for some games and still do once and a while, but there is something critical missing for me. Now like back in college when we would have LAN parties for the entire floor of our dorm, that was awesome fun. That I can really get into, but the moment a game is loaded up with some random player, something is loss and removes all the enjoyment for me.
And it isn't necessarily that I don't know any other gamers--gaming has become very mainstream in recent years. But for whatever reason, all the other gamers I know play completely different games than I do. And I've been gaming since I was 4 years old, so it isn't like I haven't played many different types of games, just some odd coincidence that my gaming lists seem to be completely different than those I meet.