iPhone 4 for me was the best release. That's why I'm still using it with no desire to upgrade at all. I have thoroughly used the iPhone 4s and 5, and I am not impressed after these 3 years. There were no major changes that interested me. Local VoiceControl without the need to use Siri is a big deal for me (only the iPhone 4 has it; although, you can put back with jailbreaking). Retina display is still great; have changed the battery for a brand new one two months ago, so I still have all day battery life - and the speed is more than decent for me even though it's jailbroken and FULL of fun stuff.
But the major drawback? I also found the sound quality from the iPhone 4 to actually be BETTER than the 4S and 5 - and it's a pretty common finding on hi-fi forums. It's something I couldn't believe when I tested all 3 out. But I guess the major reason why I'm not upgrading is the fact that I use my iPad 3 and Nexus 7 more than my iPhone. I use the phone for music, calls and messages. And music is about 85% of its use. So, upgrading will be kind of useless.
p.s. Some people in my family are very, VERY, happy; however, to be using my unused upgrades.
I would say, both the original iPhone and iPhone 4. The first iPhone, for being a brand new take on the smartphone, and making the top dog Blackberry look outdated immediately.
And the iPhone 4, for being a MAJOR upgrade, the new high resolution screen ( will never call it Retina display, that's just a silly marketing term that means nothing more, than high res ), and industrial build, was truly groundbreaking on the iPhone 4, and made Android look cheap and no comparison.
Since the iPhone 4 in 2010, Apple has made no big improvements, and has grown stale and outdated lately, The 4S was barely an upgrade next to the 4, and the iPhone5 just more of the same.
Apple needs a whole new iPhone next, #1 a revamp to iOS is much needed, and larger screen #2.
Right now, Android with Jellybean is a great OS, and will only get better with 5.0 Key Lime Pie. Samsung will hit a grand slam with the Galaxy S IV, the 1080p screen, and all new build quality with the polycarbonate body, and will be as thin as the iPhone 5, and brand new all redone Touchwiz is coming too.
Without a doubt, the iphone 4. While I currently have the black iphone 5, I miss, from time to time, the form factor of the 4(and 4s). It felt great in the hand and it was just beautiful.
The leap in performance and style the iphone 4 made is something we may not see until Apple decides to increase the width of the screen as well as make the back entirely metal. In other words, that iPhone prototype we saw a while ago.