Rank the iOS revisions from most significant to least significant.

I'm always interested in seeing how people look back on the history of iOS (which was once called iPhone OS, but for the purposes of this thread we're retconning history and just saying iOS), and which versions they think were more or less important to the development of the ecosystem and product line.

So my question is simple: How would you rank the five major revisions of iOS that have come thus far, from the most important to the least?

You'll notice I keep saying "revisions" - I'm leaving iOS 1 out of this, for obvious reasons.