This is why I still hold Halo 2 as being the best game in the series simply because of the multiplayer and what it did for online games on console. Yes, games like Battlefield and Call of Duty were already out on the PC before Halo 2 was on the console, but without Halo 2 I don't think the 360 would have been such a forward thinking platform centered around online games. Halo 2 laid the groundwork for what Halo 3, Call of Duty, and Gears of War did on the coming generation. It was basically the sole reason for the existence of Xbox Live.
Also, I think Halo 2 had the better gameplay overall. Halo 3 was still really great, but it was the tipping point where things started to get saturated with a little too much garbage that didnt need to be there. Eventually we end up with games like Reach and Halo 4 which are just so unbalanced and chaotic to the point of not being fun.
The Halo story is just a decent science fiction story and nothing more. The games always looked awesome, had interesting worlds, and really great music, but the overall narrative was never anything spectacular. I do remember Halo 2's story being nonsensical to the point where I kind of zoned out, but I could probably say that of every game but the first one. Overall I think the first game is judged on its story, and the preceding games should be judged by their multiplayer. By that mark I would still hold Halo 2 as king in the franchise.