Interesting, I read through a couple pages of this and as far as I could tell no one listed Schomburg...
1. Jack Kirby - Prolific, Action sequences are amazing, Almost single handed jump started the Hero genre in the 60s.
2. Neal Adams - Changed the game in the 70s.
3. Alex Schomburg - Crazy cover layouts that just worked. A one man Ally propaganda machine.
4. John Romita - His work on Spidey is just epic. ASM 39, ASM 50, ASM 67 Classics!
Matt Baker - His rendering of the female form may be the best in comic history.
LB Cole - His horror and romance covers are some of the most sought after in collecting. Instant classics. (See Mask 1 and Mask 2)
Frazetta - Some of the most amazing covers in the genre. If he was just a bit more prolific he'd be in my top 4.