It's quite a difficult choice. I'd go with Halo 3, but frankly it's difficult for me to actually feel satisfied with that decision, because Gears of War is such a brilliant game. In the end, I believe it all comes down to story vs. gameplay. Gears of War 1 was the beginning of the franchise and there was lots to improve, and it's story heavy. Halo on the other side, has that addictive gameplay that we all know and love, but the story is simply not the focus of the game. Again, story vs. gameplay.