If you are excluding ODST and Reach the choice of the most disappointing Halo game is obvious.
The first Halo made the Xbox a relevant gaming console.
Halo 2 was by far the best for multiplayer. It was the game that made Xbox live relevant. It was the first console shooter that everybody played online and would spend a majority of the time playing. The competition on Halo 2 match making was amazing and the game lasted years after Xbox 360 and Halo 3 was released because it was so well put together for the time.
Halo 3 was by far the most disappointing game on the series. It was a good game, and was relevant until CoD 4 was released. CoD took away some of the competition and the multiplayer was not entirely balanced. Better story than 2 but 2 had a much more lasting multiplayer.
Halo 4 is a great game. The story is good for Halo standards and the multiplayer is was more balanced than Halo 3. Games have become less about holding power weapons on your back and controlling their spawns and more focused on teamwork and 1 on 1 fights.
If you say Halo 3 was the best of the series, you make it really hard for me to believe you actually owned a Xbox.
Even with low expectations I too was really disappointed. I just can't get over how poorly done the story is and how little risk there was to create something new. As a diehard Halo fan from the beginning it's really disappointing that after the conclusion to Halo 3 and the potential there was to create something new and fresh they still have you fight covenant for most of the game. And there is zero explanation as to why your fighting them, or why they decided to ally themselves with didact. And yes I know they explain that stuff outside the game but that's poor storytelling to not have the game stand on it's own. Also it's downright silly for Chief to be in crosleep for four years and never ask anyone why the covenant are still a threat. And then the music was just super generic Hollywood music with zero soul or personality.
The Didact is a Forerunner. The Covenant worship Forerunners, seeing them as gods. It is fairly OBVIOUS why the sided with him, you don't need to read any of the books to make that connection.
Along with that, common sense tells you that a religion just doesn't die. Did you REALLY think that everyone who believed in the Covenant would just go "Oh, I guess the Great Journey is fake!" once the Prophets are dead? No. The Covenant has been around since 938 BC, they're not going anywhere any time soon. Although, the Arbiter has been trying to get Elites on his side.
LOL At saying the music has zero soul, listen to 117 or Green and Blue. --- You are the child of my makers. Inheritor of all they left behind. You are Forerunner! But this ring... is mine.