Each Halo Game has their own specialty, that's why I can't decide which one is the best of the best. However what I can say is that:
Halo Combat Evolved , which received critical acclaim, and worldwide recognition to Bungie, has the Best Story, easily understandable. It also has the Best Originality, since this game has brought huge success to sequels and prequels of the franchise, as well as the creation of its novels, and comics.
Halo 2 has the Best Campaign, featuring a wide variety of Enemies from different factions, differentiated with their rank colour. In addition, it has the Best Environment, with outstanding designs of sightseeing Forerunner structures, as well as the breathtaking atmosphere surrounding it, makes me feel like playing this game again, and again.
Halo 3 , undoubtedly, has the Best Competitive Multiplayer, since there is a huge set of gametypes we can play on, like Territories, and Grifball, bringing in new challenges to gamers upon fighting each other. In addition, it has the Best Design, with textures, shaders, and surface of the walls, armor, and characters had been given an excellent, and creative finish.
Halo 3: ODST may have not wow every Halo gamer, but Marty, and Salvatori's efforts to composing magnificent scores for this game, had brought in the title for Best Music. Furthermore, it has the Best Cooperative Multiplayer, since it has introduced Firefight, where up to 4 gamers create a strategy to defend a particluar area as long as they can, with challenges, like skulls, are arising as more sets are complete.
Halo Reach has the Best Engine, since it can carry as many as 40 AI characters, bringing in a huge firefight between both sides, making the environment of war to be more realistic. Moreover, it has the Best Innovative Design, due to the widely improved version of Forge, whereby gamers can now design their own layouts of weapons, vehicles, and structures in a more convenient way.
As for Halo Wars , I have nothing to say how special this game is. Besides, I am not an avid Real-Time-Strategy Gamer. Still, a good job to Ensemble Studios for putting all their efforts to expanding the franchise.
[Edited on 02.27.2011 5:24 AM PST]