From Halopedia, the Halo wiki
Probably Sergeant Johnson. Now I know it isn't a favourite moment but Johnson himself is what to me makes Halo a good game
Probably the Death of either the entire Noble Team (mostly Georges) or the Death of Sergeant Johnson
My favourite Halo Game is Reach, only because it was the first Halo Game I had played. Even though it doesn't really have much to do with the Halo Rings itself,It was and still is my favourite Halo Game and I still play it now.