Me being the immature little college student I am, I'm pretty sure my favorite insult is calling someone a "fuckface". I don't know what it means, but I don't care because I'm not original enough to come up with something that makes sense. However, I might just start using "S/He has the face of a bulldog licking piss off a thistle." That's genius, and as soon as I find out what a thistle is, I'll probably understand it more.
Also, I'm starting to use the word "cunt" more. I know, us Americans don't really use that word as an insult, but honestly, I don't see why not. It's kinda fun to say it, if only it didn't piss off so many people when I say it.
Update: Just found out what a thistle is. Rev's insult is fucking amazing.