The first Friedmann equation describes how, based on what is in the universe, its expansion rate will change over time. If you want to know where the Universe came from and where it's headed, all you need to measure is how it is expanding today and what is in it. This equation allows you to predict the rest!
I don't have the "most important equation" in the universe for my pick, mainly because I don't know the criteria for picking such a thing. And often times, people confuses "interesting" with "important", which need not be mutually inclusive.
It's still fun to read what other physicists think is the most important equation, even if I don't necessarily agree with their picks.