The Order of Importance
I've spent a lot of time thinking about this and the order of importance for a pizza goes like this:
Crust is the most important, requires the most effort and is most commonly bad. Its because of the three factors that come into play for this component, the dough recipe, the proofing time and the style that the pizza is cooked. If only one is missed the crust won't be right.
Sauce is 2nd most important and is 2nd most commonly bad for a few reasons, mainly because its cheaper and quicker to doctor up some pre made sauce but also because there are a bunch of dumb ideas/secrets/shortcuts swirling around about how to make sauce.
Out of the three, Cheese is the least important because its the easiest to do right. All you need to do is anti-up and buy good cheese. A LOT of places cover up a crap pizza with good cheese and think they're doing everyone a favor. (the idea of covering some garbage food up with cheese is extremely common and actually goes far beyond pizza and doesn't even require good cheese)
If I had the choice to eat a pizza with great sauce, great crust and a lower quality mozzarella or great quality mozz with poorly executed sauce and crust, I'd say its a no-brainer and go with the first option.
If your goal is to make the greatest pizza ever know, which ours is, they're all important, but there is an order of importance.