To tell you the truth, neither.
Although if I had to pick one, it would be Black Ops because of the Nazi Zombies.
I don't particularly like Halo Reach, possibly because I'm not that good at in. I always press the right stick in and accidentally scope with DMR which never happened on Halo 3. Maybe if I was better at it I would enjoy it more. However, the graphics are amazing.
Black Ops only because of Nazi Zombies. If quick scoping was still apart of the game, I would defiantly play it a hell of a lot more. Yet, I've heard rumors of a patch that will allow quick scoping.
I own both games.
But staying with MW2