A rather short but impressive singleplayer campaign combined with great multiplayer makes Halo 3 the best of the series.
Gameplay - Halo 3 is more balanced than the previous two games, since pretty much any weapon is equal and fun to use; the singleplayer campaign is incredibly action-packed and really fun above all else; blows Halo 2 away.
Graphics - Not state-of-the-art, but very nice to look at, especially the water. Some people like to complain about the graphics in this game, but I don't consider graphics to be anything important when I buy a game.
Halo 3 also has a good soundtrack, quite similar to the previous two Halos.
The story is brilliant, but I won't say anything here.
The multiplayer is superb, and has a bunch of new features and game modes that will keep you coming back.
The biggest (and perhaps the only) problem with Halo 3 is that the singleplayer campaign is rather short. I finished it in about six hours, but it's assures that those hours will be very good ones.