It's taken me a while to decide, but as I mentioned over the weekend at zhe party: I officially hate the Vahzilok the most. Sure, the 5th Column put in a strong showing, with their grenadiers and rockets. And don't get me started on the Thorns' Madness Mages. Just don't.
But oh. The Vahz. Those bastards. They put out *so* much damage in such a short amount of time, and so much of it involves their bodily fluid being forcibly foisted onto me. And combine that with their crossbows that slow me, so it's harder to deal with said puking damage, and it's a bucket of hate on them.