I had this EXACT same thread idea a few weeks ago. I could even post screenshots to prove it! I have the google doc and everything! BOOO!
Anyways, I'd say gameplay and mechanics top all. Difficutly is pretty high up there for me, even if I ragequit easily, I like a game to be challenging, otherwise it usually just bores me. My bottom two in terms of importance would probably be Music > Story. Though, music is a weird example where it being so low is actually because games tend to have decent music so consistently, that is sorta devalues the quality when the music of a game isn't absolute excellence. There is rarely a time where a piece of music just annoys me or fails to set the mood properly, so as a result a lot of decent to good ambient music in games just ends up having the same effect to me: good tone setters, but not something that I'd listen to consistently after playing the game. It's a little different though when a game almost makes it's music sort of a set piece of it's own, or if the music just sets the tone that well (Halo/Xenoblade/DMC etc). I'd personally separate sound design from music. I think it's one of the most important aspects, though.