Halo 4, to me, is the best Halo game out there. 343 tried something different by sending you to places we've never seen before in the Halo universe, and by introducing The Prometheans to take the place of The Flood.
Halo 4 was what got me hooked on the Halo universe and I absolutely love this game, from the story telling to the graphics, to just being Master Chief. I know that the story is more solid in the original Bungie trilogy, but Halo 4 is just amazing in it's own rights. 343 tried to take you to different places instead of just a Halo ring like all the other games and I think it works. There is more to this universe than just The Flood and the Rings, and Halo 4 is what gives us our first look at that.