That's kind of hard to answer - it depends on what you're looking for. If sheer abundance, June and July are great.
I've been there at all different times of year... but while summer has many benefits, it's also really hot, and often really crowded. So I tend to prefer going at quieter times of year. Then again, I really love going during the Asian Culture Festival, which is incredibly noisy and crowded, lol! So it depends on if you want a festival, a particular kind of fruit, or a quiet time when you can chat with one of the experts there without having to rush.