I think Phoenix Rising + X-Factor #2-8 would make a lovely thin OHC.

Also, Excalibur. Alan Davis' solo run would be nice in an Omnibus, but before that is slightly problematic. With all the specials, up to #43 is probably not going to fit in a single Omnibus, but breaking it in two would almost certainly mean breaking in the middle of the Cross-Time Caper, which is bad.

Samy wants to see the following collected editions:
X-MEN: CLAREMONT/JRJR OMNIBUS, X-FORCE OMNIBUS VOL. 2, 
NEW MUTANTS OMNIBUS VOLS. 1-2, GENERATION X OMNIBUS
PUNISHER (1987) #1-18 OHC,
PUNISHER WAR JOURNAL (1988) #1-24 OHC,
X-MEN: REVOLUTION OHC
X-TREME X-MEN VOLS. 1-2 OMNIBUS, WOLVERINE: HAMA/SILVESTRI OMNIBUS,
LEGION ARCHIVES VOL. 14, KNIGHTFALL OMNIBUS
LEGION OF SUPER-HEROES: EYE FOR AN EYE DELUXE HC