Buffy. She has fought more than one thing more powerful and strong than them (for example her kicking Glory's ass in the gift. Glory having super speed and super strength comparable to the Twi-vamps, in fact probably stronger with her being a God), whereas the Cullens as a group could barely manage to kill one other vampire, so going by the continuity of both stories, definitely Buffy.
This should be a pick though, the answers feature is for questions of fact, not opinion (for example asking a fact you want to know like the name or age of a Buffy actor) and also, this question would be better suited somewhere like the Buffy vs Twilight spot, since this spot is just for Buffy, and many of it's members don't like Twilight or wish to hear about it. Here's a link to the Buffy vs Twilight spot for you:
link