I think you need to separate the PC versions, especially for Halo 1. Then you also gotta consider the potential in LANs versus online and Bungie's vision versus balanced/MLG/Classic.
Things like Stat-tracking, ranking model, weapon swapping, map movement, and play smoothness goes to Halo 2 XBox-only.
However, things like consistency and weapon/vehicle sandbox balance goes to Halo CE XBox-only and LAN.
Things like Features, visuals, gameplay variety, and balance "in Bungie's vision" goes to Halo 3.
Then, there's different components like Campaign, Multiplayer, and Firefight so you need to decide how you weight those parts. In my opinion, Multiplayer has the highest weighting by far since you spend the most time there, because Campaign is like a movie you see a few times and never again. The fact that some put ODST higher up even though it didn't have a Multiplayer element is a little strange, unless you only look at the Campaign... or maybe you are considering Halo PC which is a terrible representation of the original (as seen in MCC also)
Also, nobody ever considers the game-breaking parts of the game, like out of the map glitches or lag switching cheaters / host booting and how does it affect online playability. If you take that into consideration, Halo 2 is by far the worst game. Then, if you also add the fact out of the box Bungie's vision was SMG-starts, the most unbalanced Halo has ever been (next to Default Plasma Pistol starts in Halo CE)
If I was to make the ultimate Halo game it would end up being something like this:
Halo 2's og stat-tracking
Halo 2's ranking model but Halo 3's matching algorithm or improved one
Halo 2's og aiming and movement smoothness
Halo CE og Vehicle physics and balance
Halo CE og M6D as either starting weapon or power weapon
Halo 2 og BR as starting weapon only if M6D is not the starting weapon
Halo CE og or Halo 2 Radar on
Halo 2 og Sniper, no-scope, and swipe-sniping
Halo 2 og weapon swapping
Halo CE og player movement, jump height, and crouching
Halo CE map size
Halo 3 Sword
Halo 2 Sword blocking duals
Halo CE og grenade effectiveness
Halo CE or Halo 3 Rocket
Halo 2 Flag juggling
Halo CE og Flag beatdown and carrying ability
Halo CE og or Halo 2 og Melee
Halo CE og classic settings like Flag touch return, etc
Halo 3's gametype creativity
Halo 3's polish out of the box for online playability, but better
No loot boxes or REQ packs
Combination of maps between games
Good bullet registration
I could go on and on, though.