I'd actually have to say Xander *dodges a wave of tomatoes from Fake Shemp*. Don't get me wrong, I love his character and really don't want to see any of them go, but I think that him dying would have a huge impact on the characters (create a lot of emotional stuff to deal with in later issues) and it would be the only death I can see that really wouldn't upset the story that's been set up.
Giles dying would crumble a lot of the ties Joss and co. have created for Faith and the "not at home base" characters in season 8. And especially with the amount of emphasis on his character's behaviour lately (from the Spike thing in Season 7 to refusing to help Angel with Fred to his behaviour with Buffy-- something is obviously up with the G-Man that I really don't think has been explained just yet). I think it'd also just seem forced, he's the one who, by the natural go of things, 'should' die first out of the four, which is just another reason why I don't think Joss would get rid of him. It would almost make too much sense for the Buffyverse, and I just don't think it'd have the same kind of fan reaction as Xander dying.
Willow dying would have a HUGE impact on the story that's been set up in s8 (especially with the timetravel thing), and to be honest I think she just fuels the stories way too much to kill her off. UNLESS of course, that had something to do with her turning dark again in "Time of Your Life". If they tied it in with the story, it could actually be very cool.
Then there's Buffy. It's her show guys-- come on! I wouldn't put it past the writers to kill her off again, but really without her there isn't any more BtVS. And her coming back from the dead, I think, is getting a little old. You can only play that tune so many times before it gets REALLY old, and I don't know how they could put a twist on another buffy resurrection without it seeming forced or as if we've seen it before.
So, again, Xander is where my "vote" would go, but I'm REALLY hoping they don't kill the poor guy off. He's had a pretty tough year as it is! 😉 🙁 This is all saying too that there's no chance of killing off Willow (or her almost dying) yet instead of booting her off the series have her go into dark Willow or something-- that would be pretty sweet, actually! 😀