TBH, I would say the best form factor for any iPhone would be the 3G/3GS. I mean, that's what you call a phone. Now, everything aims to be slim and easily breakable and all box-like. I mean, if you look back at the 3GS, only a few percentage of owners actually cracked their screens. Compared to the 4, 4S and 5 which has screen cracking on literally the first few days of getting it. Because it's a bit bulgy, people actually remember that the phone is in their butt pocket and won't sit on it because in the transition of sitting down, they'll actually feel it.