I can't really pick a best because I don't have a favourite, but I just want to give an honourable mention to the campaign of Halo 3: ODST. Before people call me a Halo fanboy (which I admit I am one) or decry my obscene taste in games, it's actually genuinely good. The open world atmosphere along with the audio logs and Sadie's Story allow for an interesting exploration side of things that isn't present in the main series (the closest the main trilogy comes is with the Terminals in Halo 3). The open world thing also allows a better gameplay experience, it's nice to be able to sneak up on enemies and plan out how you'll attack a force, rather than just going straight in, and if you don't want to fight you don't have to, just wait for the enemy to pass or go around, something you can't do in a linear game like the main trilogy. The fact it switches to fairly linear gameplay is good in the mission log sections, and for Data Hive, because it has the good parts of linear gameplay, and still manages to keep the open-world feel, especially in the Kikowani Station level. That bit also did aerial combat really well, I think, and made for a much more interesting bit of gameplay. The level design is also great, and the characters are actually pretty likeable and cool (though admittedly any group of characters voiced by half the cast of Firefly will be pretty cool...).
Overall, a great game with a great campaign.
