That's simple; Firefight. Reasons:
Warzone was a great idea, but it fell short of what 343i probably thought it would be. On paper, it sounds like the greatest experience in Halo; two teams of 12 players fighting to control key points to unlock the others base so they can destroy the core and win the game? Meanwhile having to battle tough enemies that spawn up at certain intervals, forcing both teams to either take advantage of the others distraction or work together to beat the common enemy? Sounds like a battle of epic proportions! Except that's far from what it ended up being; it quickly went from chaos to the bomb rush it is now where both teams try to rush the opponents bases the moment a high tier boss pops up. (If they don't try to do it right away. Well coordinated teams with skill can cap all three points and then converge on the opponents base, spawn camping the doors so no one can get out as they push in to win. A legitimate strategy, sure, but it doesn't make for very thrilling game play/experience. I say this having been on both ends of those fights. Steam rolling leaves little to be done and being rolled is frustrating to the point I honestly don't want to play.)
Warzone Firefight was a step in the right direction, but there's so much wrong with it that actually makes it downright infuriating to play at times. Like for instance the short timer for each round, you get 5 minutes to defend a thing, destroy a boss, or take down a wave of enemies you probably don't have the Req level to get the weapons to do so efficiently (we're looking at you Round 2 Banshee wave...don't think we've forgotten about you so easily...). Another thing is the constantly increasing respawn timer duration. Accidentally got run over by your team mate fleeing for his life (or trolling, whichever) at round 5? Too bad, gotta wait half a minute to get back into the action cause Palmer/Buck/Exuberant Witness/Yabda the Merciless says so. Because that's efficient. Hope you didn't spawn a rare and/or powerful weapon, because it's probably gone by the time you've realized what just happened that brought you to death. (Don't even get me started on the broken animation of spawning a vehicle from the launch bay, if you or the host lags even for a second while coming out of it, you will die for no reason.)
Do we even want to talk about the god awful spawns for most of the Warzone FireFight maps? Half the time you spawn on the opposite side of the map on foot. I get not wanting us to be spawn killed but please move the spawns closer to the action if Warzone Firefight returns to Infinite.
Invasion was also a neat concept, except more often than not, the games were one sided. This was because of consistent spawning which allowed the better teams to literally spawn camp the opposing team with their own arsenal provided. If the Elites manage to establish a hold, they have two people stay back and guard, while the others rush out and spawn camp the Spartans. If the Spartans manage to push, the Elites would start spawning where the Spartans did and would have to run all the way back only to lose their base (I think this was a spawn bug, as when I went into Forge on an Invasion map, there were no Blue Spawns where the Spartans spawned. So, likely having the enemy team all over the Elites base, the game defaulted the Elites spawn to the Red team respawn points to avoid spawn killing since proximity spawn negation was a thing.) If they bring it back, it has to have its own set of specified maps. Using pre-existing maps only leads to chaos and makes for poor game play.
Firefight was arguably the best out of all of them because it offered fun game types like Fiesta to the weapon loadouts. What will you spawn with? Who knows, but you'll have an unlimited ammo source for it so go nuts. It was a seemingly endless game type that gradually got tougher and tougher as each round and wave set went by. The most thrilling aspect of it was seeing how far you could go. This is where Warzone Firefight fell short because it's only 5 rounds and you only get 5 minutes each round or else you fail.