Logical Answer: Mosquitoes, as they serve no environmental function other than spreading disease.
Illogical/Irrational Answer (would have been done a few years ago): Fire Ants (Can't believe no-one's mentioned them yet), I'd had a semi-justified hatred of them for as long as I can remember, as they'd repeatably turned up in playgrounds, front/backyards, and bus-stops throughout my childhood. Although, the hatred is probably my mom's fault above other things. Apparently, while my mother was growing up in Illinois, she'd always mess with ant hills for fun (Warning sign anybody?), and then, when she moved to Texas and had me, she supposedly saw a huge ant hill out in the front yard, sat me down, and encouraged me to poke it repeatably with a stick, give you two guesses what happened.
For the record, I'm over the fire-ant thing, I've realized that they were just doing what they're supposed to do while ALSO serving a vital role in the ecosystem...unlike mosquitoes...