Probably Jasmine.
I mean, I liked The First more and thought it was a cooler villain (mostly because I just despise Jasmine), but Jasmine was actually succeeding at her goal. With The First, it just kind of walked around and talked for ages and ages, make some massively huge mistakes, and then lost a straight up battle with a horrible plan.
I mean, just thinking logically as opposed to as a fan, the wave of uber-vamps was intimidating, yes, and I'm sure they would have done some damage, but does anyone really believe that they would have taken over the world? Come on now. For starters, one American military fighter jet could have taken them all out, when you think about it. They're not going to be able to cross the Atlantic Ocean (unless they have some ability to transport themselves between Hellmouths, which they very well may), and really, once the U.S. and Canadian militaries become aware of what's happening, they're going to wipe them out pretty quickly.
I think The First, as dangerous as it was, gets blown out of proportion more than it should.