Halo had its time. Since it's only on Xbox, kind of exclusive.... CoD just blows, zombies is fun though. GoW is simply amazing. I almost felt like crying at one point in the second game.
I have to say Halo is the better of the 3. I've played all 3, and must say they each have their own strong points. I like Halo, because it's so much more than a game, there is a whole comic series on the Lore of Halo extending past Halo 4, that's how the future games come to be. Aside from the Lore and comics, Halo has a pretty awesome campaign storyline (If you payed enough attention, all the little details go together) Something that both games lack, GoW less than CoD is love AFTER the game is released. Halo 4, every week since it came out released 5 Spartan Ops missions, added a new playlist in Multiplayer, and do Daily Challenges to earn more Credits. I know Gears of War does something similar, but CoD from what I have seen, does nothing in the similar. That's why my vote goes to Halo.