I think Mt. Hamilton from the west as the easiest. I've done Diablo many times from the north or south several times and, although they are the steepest, I found Mt. Tam from Fairfax the hardest. I think that it's because it's the longest and the total ascent is the greatest. There's like 1200 feet of descent on the way up to the top, which of course turns into ascending on the way back. I also think it's prettier than Diablo.