Fantastic, thank you. I think I'm going to go with that Animal Man omni. I have Arkham Asylum and it's so cool to see what he was doing there and how similar themes have carried all the way through his Batman run. This and the prose he wrote from the Joker's perspective during later Batman stories (I forget which trade I read it in) are like study guides to understanding his take on the Batman world.Animal Man is a must, and the omnibus is small, beautiful, and contains the entire run, so it's worth the money.
Seven Soldiers is excellent and truly is a major event that is self-contained and stars unknown heroes. Read it if only for Klarion the Witch Boy. And Frankenstein.
Arkham Asylum: A Serious House on Serious Earth is a must-read if you enjoy Morrison's Batman stuff. It's his first Batman-anything, and holy fuckballs, is it great and boundary-pushing...
Also, his Supergods book is like a cheat sheet for reading his stuff. He explains how he sees the different heroes and I found it really helpful understanding his different takes on Superman. I can't wait to read his Earth One Wonder Woman eventually, in Supergods he talks a bit about her and he has a fondness for the way in which the character was inspired.
Thanks again for replying to my trade question everyone!