I think X-Factor #1-8 is perfectly fine to collect on its own, remember that there's the setup in Avengers and FF, the first issue is double-sized and there's at least one annual that takes place before Mutant Massacre. Not every OHC needs to be 20+ issues! When the story breaks warrant it, there's nothing wrong with a dozen issue OHC.

If we're going to OHC PAD's X-Factor, I think it would be a crime to skip #87, his most celebrated issue. I think collecting his post-Song issues and Annual ? (#8?) would allow for the completion of the Chalker brothers subplot which doesn't reach a satisfactory conclusion if you end at #83.

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