(I can't believe that this thread has been around since last September, and I've not spoken my piece on the subject yet, so, here goes)
Not surprisingly, I cast my vote for Spike, because I honestly believe that he is what Buffy needs right now. A boyfriend who is as superhuman as she is, who she can trust implicitly to always take her side in any argument and to stand at her shoulder or cover her back, no matter what she might face. That's exactly what she needs RIGHT NOW.
But someday Buffy's going to pass her stake on to Faith, Satsu, or some other worthy young Slayer, because she's going to be the first one to get out of the battle alive. Then, she's going to need someone who can give her children, can grow old with her, and can simply BE with her throughout the peaceful time. When that time comes, Spike's gonna go back to that African cave, pass another set of trials that would be impossible for any other man, and William Pratt (or whatever his last name really is) is going to marry Buffy and they're going to live happily ever, despite Joss' best efforts to gum everything up.
(Eat your heart out, Bangels)