Halo CE 2v2 is generally understood to be the best competitive Halo. Its quickly spawning power weapons on predictable static timers force players to move and think ahead, and its spawn system gives you very precise control of your teammate's spawn if you understand its inner workings and know the maps. On top of that it has a plethora of small tricks that spice up the game, and it has the fastest gameplay of all Halo games.
It's specifically 2v2 that is held to high regard. 4v4 isn't as popular, in part because with the increased number of teammates you lose control over the spawn system. This is completely unique to CE, and in the rest of the games 4v4 is the more popular competitive mode. CE is altogether very different from the rest of the games because the individual is very powerful in it. So, whereas in the later games you have much more direct reliance on your teammates in the form of team shooting, in CE the teamwork can be more indirect.