Now you're all going to say that this is a cop-out but .......
As all our instructors told us, I'm sure, "If you fly a controlled (or even perfect) circuit you have a much better chance of a making a controlled (or even perfect) approach. If you fly a controlled (or even perfect) approach you have a much better chance of getting the flare right. If you fly a controlled (or even perfect) flare you have a much better chance of getting the hold-off right. And if you fly a controlled (or even perfect) hold-off, you have an excellent chance of completing a perfect landing."
So I'm not sure that any phase is more important than any other phase but they do all rely on each other (as feetdry has pointed out). Of course the nearer the ground you get, the less margin for error. Which is why I prefer to see a student make an early decision to go around rather than trying to "rescue" what we both know is turning into a messy approach. But I'm only one of a million instructors so let's wait and see what the others think !