Good thread, and a list that's impossible to prioritize; it's like asking, "What's your favorite album?" The answer changes on any given day.
That said, my top 10 (in no particular order)
---Jack Kirby (there's a classic two-page Forever People spread that has stuck in my mind's eye since I first saw it; Evanier's new book is must-reading)
---Jim Steranko (his work on Nick Fury just blew me away; way to blow a pre-teen's mind wide-open, Jim)
---Barry Smith (watching his style develop over just about two years of Conan was fantastic; to this day, I have his massive print of Pandora framed on my office wall)
---Gene Colon (what he did with shading; I love his work on Daredevil)
---Steve Ditko (I've never quite been able to put my finger on it, but something about his style really engages me; his Dr. Strange and Mr. A work more than Spidey)
---Will Eisner (just wonderful, whether in Central City or on Dropsie Avenue)
---Neal Adams (I really dug his work on Green Lantern/Green Arrow and X-Men)
---Harvey Kurtzman (the EC war comics in particular)
---Frank Frazetta (I'm hesitant to include him, given his relatively limited volume of comics work, but Thun'da and some of his cover work and those EC stories are just so damn good)
---Basil Wolverton (adding his name in here is like including some weird novelty song in your Top 10 Songs of All Time list, but just try to not be fascinated by his stuff!)
I'm tempted to try and kick someone off the list and stick Jim Starlin in there somewhere, but I just can't do it (sorry, Jim) ... John Romita Jr.'s work on The Eternals is some of the most striking stuff I've seen in a long time ... Todd McFarlane is brilliant ... and as long as we're talking artists: Why does Gil Kane always have to look up everyone's nose?
As a relative newbie to the site, I'm looking forward to exploring many of the great Golden Age artists in more depth; Starlin may have some company in my "bubbling under the top 10" list a year from now.
Cheers, all!