Halo 3 is my personal high point of the Halo franchise, though ODST and Reach were both good, neither of them were great whereas the original 3 games never sank below greatness. Although Combat Evolved is the one that is remembered as the innovating, genre defining behemoth and classic of the series 3 for me is better in almost every single way.
Halo 3 was a masterclass in game making, the superb first person gameplay is matched with a fantastic story, memorable characters and locations, great graphics and one of the finest soundtracks in gaming history. The single player is an exhilarating thrill ride that never lets up and this alone is enough to cement the game in it's legendary status but this is only the tip of the iceberg.
The single player campaign is coupled with both split-screen and online multiplayer and there is both solo online and split-screen online competitive game modes and of course Forge mode, a mode where you can create your very own sandbox to wreak havoc in either by yourself or with a group of friends.
Though I may have abandoned the Xbox 360 last year due to my loss of faith in Microsoft (that's really a whole other story which I may address soon) Halo still remains an awe inspiring franchise in my eyes, one that I love and follow with great enthusiasm and one that I hope will continue to excel for many years even though, in my eyes, Microsoft don't deserve it to. Again though, that's a post for another time.