While its fiction can be a little uninspired, Halo 3 shows off some of the best level design and gameplay anywhere.
Once you start Halo 3, however, it quickly becomes obvious Bungie have outdone themselves and nearly every other FPS developer on the market. The scenes and landscapes you fight through in Halo 3's single-player campaign are amongst the most epic in the genre. Not just in terms of scale, but in terms of immersion and playability. Navigation has become so bad in games that many developers shove floating arrows on top of your avatar just so you know where the heck to go. Halo 3's levels feel like authentic environments, but Bungie has done a superb job of signposting progress so it's never a pain to simply move around. These are iconic, memorable levels -- storming through the mechanical and organic guts of the Flood's base ship, sneaking through a jungle in the game's opening mission, an epic finale in the sky. The single-player campaign is nearly like a Greatest Hits package, filled with a batch of replayable levels you'll want to dive into again and again.
As good as the gameplay mechanics, art direction and level design are, it's disappointing that Bungie hasn't invested as much time or resources (or talent?) into really developing Halo's fiction or characters to the level seen in other AAA series like Grand Theft Auto or Bioshock. At its heart, Halo is still a homage to Aliens or Predator or any of the dozens of Hollywood sci-fi action films that have endlessly been copied and re-copied over the decades. It's partly a criticism of games, in general -- story and character nearly always take a back seat to graphics and gameplay, even in single-player focused games. But with a franchise this famous and a developer this well-financed, you'd hope they could up their game. The fiction of Halo, to me, is science fiction at its worst. Shooting invaders, flying spaceships, saving the world. It's Saturday-morning-cartoon science fiction. Even lesser-known games like Dead Space are doing more interesting things with storytelling and character in a space-themed shooter. That's not to say Halo 3's campaign is terrible. It's not, and it's the best in the series so far.
I'm not a big multiplayer guy, so for better or worse, I won't touch on Bungie's genre-defining work. It's obviously among the best in the business, if you're so inclined.