The question is difficult to answer because of what it means in terms of changes to the arc. If the show ends with The Gift, that means the channel switch (and man, those were heady times in the fandom) never happens and the original plan for season 5 likely goes into effect (which you can read about over at the Buffy wikia). The major plot point of that to remember was that the destruction of Sunnydale happens then instead of at the end of Season 7. In that case, no character would be staying in Sunnydale cause there was no Sunnydale. So then... lets take a look at the characters in that light.
1. Buffy- Not likely. Even if she didn't die (which was not always the case), SMG had already threatened to quit if Buffy left the WB (no, really). Since she was already against, we should go over to a spin off after "her" show got canceled? No way.
2. Willow- Very likely. If the original plan went forth, she'd have been Dark Willow now after Tara's death earlier, and would have joined Glory in fighting Buffy. She was never likely to be killed and would have fit right in with the redemption aspects of Angel. This was actually a popular rumor back in the day before the switch was known for sure, at least among the fandom. It would have be interesting, however, to see how her character would have bumped into Cordelia on Angel. They kinda seemed to fit the same purpose.
3. Xander- Almost impossible. Didn't fit the style or the series, was already covered by Gunn's character really (though that would have been interesting to see) and would have been dead given that the original plan was for him to be the other half of Glory. Meaning Giles would have killed him.
4. Giles- No chance. It was already well known by midway through the fifth season that, no matter what happened with the channel switch, ASH wasn't really interested in doing another season, so no way he'd head to a spin-off. In fact, at the time, many thought he would die, and that spear he took seemed to confirm it at first.
5. Tara- No chance. Again, she'd be dead if the original plan played out. Not to mention her character never would have fit the Angel concept.
6. Anya- No chance. Again, dead in the original plan. (Yes, the original concept for season 5 was this bloody to the characters, way bloodier than season 7).
7. Dawn- Depends. Whether or not Buffy or her would die, and there was a lot of discussion from both (or so it appeared at the time). If she lives, then its certainly possible. A storyline of her seeking shelter to mourn Buffy with Angel was another rumor making the rounds at the time. I could also imagine Michelle Trachtenberg signing on to that despite her loyalty to SMG.
8. Spike- Almost certainly. The most obvious, really. The rumors at the time were that Marsters was the most tweaked by the possible channel switch/early cancellation. He was still the new kid on the block as far as the main cast went, having just been added half way through season 4, despite his many guest appearances in the past, and now that his character seemed to be getting a meaty plot arc (his "interest" in Buffy) it seemed to be influx. Marsters definitely would have followed Whedon over to Angel, and probably would have butted heads with Angel. It was suggested that maybe the plot there would be he followed Dawn to LA, and in the process of protecting her as a last request to Buffy, become involved with the Angel gang. But this never was more than rumors.
So, yes, Spike and Willow make the most sense, but that is largest cause their characters were the only ones who may have been left standing and their actors the only ones willing.