First of all... Buffy/Spike = Mother of BeJesus no!
It's sick and wrong, as already said Buffy neede dSpike to fill a void in her life, she doesn't love him just cares for him!
Buffy/Angel = Maybe... they were in love but I think they've both moved on
Buffy/New guy = Ahhh... no! I don't think so, Buffy doesn't need another, it'll just complicate things even more!
Ahh... but what about Buffy/Xander = why not? They have been friends for a long time, they understand eachother and care (and possibly love) eachother
If not Buffy/Xander I say Buffy/no one = She has friends who she loves and who love her back, if not Xander I think she should end up alone... well not completely, she has a loving sister and loving friends! And to be honest that's what I think will happen... she'll end up alone... although not completely alone 🙂