I really would like to see the following omni:
1) Amazing Spider-Man by Michelinie and Larsen (including Larsen's "adjectiveless" Spider-Man run - or, maybe an omnibus of McFarlane's and Larsen's "adjectiveless" SM run)
2) DeMatteis and Buscema's SSM (maybe with Kraven's Last Hunt thrown in)
3) Thunderbolts by Busiek and Bagley
4) Avengers by Kurt Busiek
5) Invicible Iron Man by Matt Fraction and Salvador Larroca
Honorable mentions:
- Punisher MAX by Garth Ennis (I own the OHCs, but would gladly double dip this one)
- DeFalco/Frenz ASM (with the recent Epic, this one looks very unlikely)