Halo 4 was the last one I played, but even Reach was kind of a let-down.
Halo 4 wasn't Bungie and it was worse off for it. It almost played like Call of Duty, which I would buy if I wanted to play such a game, but I want to play Halo, which typically controls similar to a 90's shooter. So, that was annoying to some degree. The campaign was pretty boring as well. Canyon Runs are far from my favorite pieces of any FPS. The story to it was kinda meh in general as well. Killing off Cortana, which was a huge appeal to tons of gamers everywhere (Just look up how much porn/lewds there are of her) kills off a large part of the franchise.
Reach was disappointing because it took major story elements from the first Halo book "The Fall of Reach" and sorta threw them in the blender and then tossed it on a disc. The book was better than the game, although that's kind of typical, this was a real disappointment. Didn't even keep the names, one bitch got snarky about Master Chief's size, "I thought you'd be bigger", was her comment.
At least ODST was playable once you realized you weren't a spartan anymore and were actually pretty squishy so it required you playing more tactically.