I'm not saying I'm settled on anyone being the father, but been thinking about it quite a bit, and to be specific, about why Spike could be the father. Could be, not it:
* Spike was around lately, and he and Buffy seemed to get even closer, I've seen many comic shots of them together, they had many scenes together lately. They just seemed very close friends lately.
* The cover of issue #7 that shows Buffy and Spike having a normal life, sitting on the couch together, and enjoying their time as normal people. Could be misleading, yes, but even if it's just a dream, it suggests something real, that might have something to do with a little earlier issue and what has happened in them.
* I think it was Scott Allie who clues to Spuffies that they're in for something good.
* Spike is supposed to have a bigger role this season, and have something to do with Buffy.
* It's been mentioned that Spike wouldn't use Buffy when she's drunk. But at that party everybody seemed to be a little drunk or under a spell. Spike even spoke to a girl (who wasn't Buffy), so it might even have something to do with that.
Why I doubt Angel is it:
For starters, he has his own comic and he's away, so he won't even be around to see his kid. Also, Angel already has a kid, so I doubt they will go with it again, more kids for Angel.
I however, don't exclude him compeletely over the fact that he's a vampire. That's not a barrier in this verse. Therefore, it's another reason why Spike could be the one. But not a reason to exclude Angel!
My point is, as far as my opinion goes, Angel could be the father. But there's a bigger chance that it's Spike. BUT there's a better chance it's neither. But out of these two, Spike has a better chance to be the one.
Getting to think about it some more, I think there's a big chance that it's not Spike or Angel, but Spike will be the one to raise the child with Buffy.
Just my opinion.