I absolutely loathe August and November. August is way too hot and humid, and November is when everything starts to die off and looks really depressing and sad.
I also really dislike February, since that's when it's coldest/snowiest/iciest here and it makes it hard to go anywhere.
I don't like February and March. In the southern Hemisphere that's when its warmerish and stuff, but its coupled with the fact that the excitement of talking with my colleagues and team at work has worn off and I get back into the boring routine again...
I'd probably say July. I tend to prefer cooler temperatures over warmer temperatures, so I favor Autumn and Winter over Spring and Summer. As someone without an air conditioner, the Summer heat is sort of a bother. I wouldn't say June because a few of my friends have birthdays during that month. August could be a contender too, but at least in August you're closer to the arrival of Autumn, so it's more tolerable. Other than the 4th of July and people shooting off fireworks, July's just hot and there's no end in sight.
It's too hard for me to pick. April through May are depressing for me because winter and nice weather are quickly coming to an end. June through August are ridiculously hot. September, as well (and sometimes October, and November if we're really unlucky).
I'd probably go for April or May. They're usually too hot for me to begin with, and they're full of severe thunderstorms. June and on may be hotter, but as I said, April and May are hot enough.