I have no source on this, but it has always been my understanding that Dayenu is simply a poetic build-up to the next paragraph:
Thus how much more so should we be grateful to the Omnipresent One...
That is, it builds a nice chronological list (not priority order or there would be fewer answers to this question) of all the great things G-d did for us. It says, "Even if He brought us from Egypt it would have been enough. And He did that! If He brought us from Egypt and carried judgments against them, it would have been enough. And He did that, too! Awesome!" etc.