3G, 4, and 5.
Would not have upgraded to the 4, but opened before selling it and was hooked on the beautiful screen and fluidity (speed) of interface and apps over the 3G.
Would not have upgraded to the 5, but people stopped being able to hear me on calls with the 4, and I desired the new aspect ratio of the screen.
Almost upgraded to the 5s just for the gold color and LTE (using a Verizon 5 on AT&T, only 4G), but could not find any gold in stock around me at launch, and my interest subsided later.
May be upgrading to the "6s plus" because of rumors that AT&T could be discontinuing 2 year contracts, and would like to lock in my upgrade.