Not a particularly uncommon one, but wasps - and by extension, any buzzing insect I can't immediately identify.
Flies, mosquitoes, earwigs, spiders, any other creepy-crawlies aren't a problem once I've identified them as "not wasps". Even bees - if I can tell that the thing flying around is a bee rather than a wasp, I might be a bit jumpy but I won't freak out like I would with the devilbugs.
Think it might be because bees only sting you if they really have to. Wasps sting you because fuck you.
Of course, logically this whole thing doesn't make a lot of sense - phobias, by definition, don't - and it annoys me that I can't help but freak out if I'm so much as in the same room as something so small and... well, not harmless, but definitely not deadly either.
I mean, I'm a chemist. You'd think I'd be more scared of concentrated acid or dimethylmercury or something.