iPhone 3G for me - although in the past few months I have been very unhappy with it. Apple rolled out a firmware update (iOS 4) which has slowed my iPhone to a crawl... it was almost unusable at first. I really can't see how they tested it and thought it was satisfactory to release the update. All my friends with iPhone 3G's have had this problem too and it is a widespread issue on the net.
The new 4.2 update should hopefully fix this (or at least improve the situation), but it has left a real sour taste in my mouth and put me off buying Apple products for some time. It's a real shame, as I loved my iPhone until it slowed to a halt. For non tech-savvy users that wouldn't be able to downgrade the firmware, it would be a real kick in the teeth.