It would basically depend on what point in the show's chronology you are talking about. In Season 3, Faith and Buffy were pretty close to being equal in skill, and either Slayer could probably have taken either vampire, though not easily. Also at that time, Angel could take Spike easily, though that was more due to a lack of confidence on Spike's part than any superior skill on Angel's. Angel had spent a century intimidating William, and he did a very good job of it. So, by Season 3, the first time that all four characters existed:
1) Buffy
2) Faith
3) Angel
4) Spike
By the end of Season 7, Buffy's skills and experience had improved considerably, while Faith had been either in a coma or in prison, pretty much out of any real action for 4 years. Both Angel and Spike by that time have been growing in skill and experience, but, in addition, Buffy had been encouraging Spike, convincing him that he could be, and indeed WAS, a better man that he had previously believed, building his self confidence in a way that no one ever had before. During this time, Faith did beat Angel on his own show (though more by subterfuge than actual skill, injecting herself with vampire happy juice then letting him feed from her) but she couldn't beat Spike either of the two times that they fought (though it must be noted that they were interrupted before any conclusion either time) so by Season 7, the ranking would have shifted to this level:
1) Buffy
2) Spike
3) Faith
4) Angel
with Angel and Faith so near in skill that their relative ranking could go either way.
By the end of Season 5 of "Angel," Spike had not only managed to beat Angel, but he has also attained a skill level sufficient to hold his own against Illyria until she brings her time warp ability into play. In his own words, sparring with Illyria allows Spike to "sharpen up techniques that he didn't even know were rusty." By the end of Season 5:
1) Buffy
2) Spike
3) Faith
4) Angel
with Buffy and Spike having almost equal skill levels, so that neither one would be clearly superior at all times, and the same thing being true for Angel and Faith. (Granted, we see neither Buffy nor Faith during this season, so my assessment of their skill levels is based more on conjecture than actual knowledge, but I still feel that it is valid)
Season 8 and beyond would be anyone's guess, since we never actually see any sort of match up that would be indicative of improvement for any of the four characters. The fact that Angel was able to beat Spike in their one fight proves nothing, since Angel was possessed by Twilight at the time. Therefore, with no new evidence, or conjecture, to go on, I will say that the final ranking as of this time, remains:
1) Buffy
2) Spike
3) Faith
4) Angel
with Angel and Faith being too close to definitely tell.