Probably July, with August as a runner-up. Growing up, July was the first half of summer (we got out in late June), and in years where we went back to school before Labor Day, it was the only month that was entirely summer. In high school and college, assignments and commitments and such would begin to build up in mid-late August, so that part wasn't as fun. August was frequently when I took a vacation, though, so I liked it for that reason, though I hated seeing summer come to a close.
This July hasn't been great, and the distinctions aren't as clear-cut as they used to be, but things still feel more relaxed in the summer, and it's easier to get outside and do things. The one thing I dislike is the heat, but now I spend more time in air-conditioned buildings during the summer than I used to, so that's less of an issue than it used to be for me.