Ears. None of this really matters without 'em. :)
Second most important is room.
No system will sound good in the wrong room I don't care what components you have.
Third - When it comes to electronics, speakers are the most important. Yes.. yes.. I know.. garbage in, garbage out but if you have a great source and garbage everywhere else, you still get garbage doncha? Speakers establish a dimension of possibility within a system - good speakers with garbage components can still sound nice (with the right cables) but... take a pair of crappy speakers and hook them up to any system you want and it ain't going to sound pretty.