iPhone 1 (2007) It died about 2+ years after I got it, and I bought my current 3GS.
FYI a friend of mine who just got an iPhone at the end of the Unlimited period a few years ago but he got to keep his Unlimited plan. When he went to AT&T to order the 4S, it tried to switch him out of it of that plan, but at Apple.com he had to choice to keep it.