Halo has a definitive edge when it comes to graphics, animation, sound, and core shooting mechanics. It also has online co-op and multiplayer, and I personally really enjoy Halo's multiplayer and it's a huge plus to be able to play the game with your friends, although I know a lot of people have different opinions on its multiplayer.
Fallout's on the other hand has poor graphics, animations, poor AI, and less than stellar shooting gameplay, but despite this, with its roleplaying mechanics, loot collecting, and open world, somehow it all comes together in a great package that manages to overcomes a lot of these issue.
Still, I'd give the hand to Reach.