Yeah, Halo 3 probably had the most iconic maps of the bunch, in my opinion. Guardian to this day remains my favorite, I think. With things like Sandtrap, Sandbox, and Valhalla (Ragnorak) not very far behind. Oh, and the Foretunner map with the man cannons, that one was great too.
That being said, Halo 4's maps are in no way bad, and I absolutely love the additions to the gameplay.