Invasion as I like the UNSC vs Covenant Dynamic. I do hope we can get some NPCs in invasion though, maybe some Marines/Grunt/Jackals to accompany the playable Spartans/Elites.
Dominion seemed like a good concept but terrible execution. If dominion had a UNSC vs Covenant aesthetic it could have been better. I would love to destroy a covenant base, capture their LZ and watch it turn into a UNSC Camp as Pelicans drop barricades and bring NPC reinforcements for the garrison, or visa verse.
Warzone gave us NPCs but it had two problems, One the NPCs looked goofy (1st we had Marines, then we had troopers, now we have Go Go Power Rangers), and the Req system which essentially was pay4better weapons/vehicles/damage/health (essentially pay2win).
Here is a crazy thought, give me the UNSC/Covenant match up from Invasion, the base capture and upgrade mechanics from Dominion, and the (redesigned) NPCs from Warzone.