What is the most competetive Halo Game?
Interesting question, probably Halo 2.
The game was far more competitive when compared to Halo CE.
Button combo’s existed, which is similar to some of the tech’s performed in Melee.
And there existed a large community who played Halo 2 even when Halo 3 came out. The Noble 14 is a example of the dedicated fanbase too.
The next game, Halo 3, is somewhat similar to Brawl in the way that it was more casual and sold more copies. Not only that, but people have a ton of nostalgia for it and it was their first dip into the world of Halo.
To answer the question of the title, Halo 3. To answer OPs question, Probably Halo 2 or Halo 5.