I've always run 235/75-15's (as shown in the attached pic), but will either go with a 255/70-15 or a 275/60-15 next time. But one thing to consider is that the 235/75 offers TONS of tire choices, whereas the offerings for the other sizes are very limited.
I always consult tirerack.com before making a tire decision. Lots of good comparison info there.