Absolute Crisis on Infinite Earths - my favorite story with a bunch of extra features
Absolute Long Halloween - another favorite story by the Loeb/Sale team
Absolute Kingdom Come - another favorite story, this time by Waid & Ross
Captain Carrot & His Amazing Zoo Crew - bound volume containing the entire series plus the Oz-Wonderland War.
Amazing Spider-Man Omnibus - makes for great reading and doubles as a nice blunt weapon if needed