I like to look at it from a slightly different angle in thinking that no one single thing is "the most important," IMO there is no most important anything when it comes to audio. Acoustics are important, monitor quality is important, preamps are important, mics are important, hell even the musicians and their gear is important...but not any single one of those is more or less important than the other. It's all important, any sub-par link in the "chain" is a weak point. So $10k monitors will suck in a bedroom with no acoustic treatment. Your U87 you just dropped a fuckload on doesn't mean shit if you are using a Behringer preamp. I can go on and on. But you guys know this.
~006