4
No other phone in the history of phones would I have been willing to line up and wait for outside the store to get one.
Those who remember will recall there wasn't a pre-order process like we have now, and the 4 launch was handled poorly. You could "reserve" one through your iTunes account, but it didn't require a CC or deposit, and no one knew if they were actually going to get one. Best Buy took pre-orders too (a buddy of mine and I were #1 and #2 in line), my friend ordered a white one (remember THAT ordeal???), and I ordered a black. Because White never arrived, I actually got the ONLY phone Best Buy received, and my friend ended up with nothing lol...
The 4 itself? The best phone I've ever owned (and I've owned a lot). It was a marvel. Especially during the first few weeks it generated attention unlike anything (even the original iPhone). Everyone wanted to check it out. It's screen was an advancement the degree we'll probably never experience again. The A4 was a powerhouse in the iPad, and here it was in my PHONE.
The 4 is also the phone that eventually ruined the iPhone "cool factor". It was SO good that it started to infiltrate every part of society. Soon, even retirees and technophobes had one. The iPhone went mainstream.
There was actually a time when having an iPhone was a status symbol. Not anymore of course, but man was it fun back in the early days...