Halo CE was a flawed masterpiece, with sometimes questionable design (the Library... Oh god...)
Halo 3 was really good, but when I was done with it I really felt let down. It was almost anticlimactic, with too many portions almost literally taken out from the previous entries.
Godlike Multiplayer though.
ODST was alright, it was a fresh and new vision of the Halo franchise, but nothing groundbreaking.
Reach is my personal favourite (but not objectively the best). The way it takes you on its ride, even though you know exactly what's gonna happen, you can't help but hope. It was darker, grittier and more intense story wise than any other Halo in my opinion.
And the MP was excellent.
Halo 4 is a little weird. Disappointing multiplayer, not to say "poor", but great single player campaign. I really enjoyed it, and it took Halo in an interesting direction.
Halo 5... Great multiplayer, with a lot of competitive potential. But the campaign ? They didn't even care about it. Poor story, some poor characters, awfully short, too easy, with no moment to really remember. It's just frustrating to no end.
One of the worst of the franchise (sorry) in my opinion.
And then there's Halo 2. It might not have aged excellently, but it is the very essence of a genre defining, groundbreaking game. An immense Single Player mode (but with a kinda disappointing ending, I must admit), with cult moments, great characters, challenging levels, etc... It's just a masterpiece in that regard.
And the multiplayer is simply phenomenal. One of the best, even to this day, and set the standard for all console FPS at the time.
Simply a historical game.
Just because you have an opinion doesn't mean you are necessarily right.