Halo 2 is the best simply because it was revolutionary and simple.
The online system revolutionized console gaming at the time. It's still the best Halo that game out because it had the perfect blend of complex and simple gameplay. None of the equipment or AA's that slowed down gameplay. It was a simple, fast paced FPS. You had someone shooting you? You better turn call out to your teammate or turn around and out-shoot them.
Melee was better because it wasn't just the double melee bull-blam!-, you actually had to have skill and press B-X-B faster than the opponent, so the fast fingers won melee fights.
Also, I don't even need to go into the fact that the maps were very well built, from the small to big maps. And how the ranking system was extremely better than Halo 3 or Reach.
But one of the main reasons the game was so great was because of all the glitches. There were whole communities made of up glitches, super bouncers, modders, and legit-players.
SuperBouncing was the greatest mistake ever made by Bungie because it spawned a ton of sub-culture teams that used bouncing as a way to compete, see who had the best bounces, who could hit the most double/triple bounces, etc.
So many things to do. If you were bored of playing MM legit, go play fun customs like Warthog Wars/Troy/Zombies/etc. Bored of that? Go glitch and find some new ways to get out of the map or duplicate a warthog. Bored of that, go work on some superbounces.