I'm the opposite of most here.  My first preference is Essentials, followed by classic paperbacks, then Masterworks & Premiere classics.  I work down the line based on the probability of something being essentialized or not.  Price, a late start, not caring about the 'shelf look' and not being bothered by b&w are my motivations. 

I have to admit aesthetically that Masterworks are far superior to Essentials and there are certain artists I need to have in color (Starlin, Steranko, N Adams).  I'm also beginning to break down on the 'shelf look' nonchalance.  I had an opportunity to get the limited cover Venus but decided to hold off and get the regular version because it looks nicer with those other bronze books.  Otherwise I have both versions of Premiere classics & Essentials, 3 versions of Thor Simonson visionaries, etc.  The Earth X series is in 4 different formats.  As 'Shelf Porn' this ranks up there with Moe from the Simpsons and his Sears catalog.