Call of Duty 4 followed closely by Call of Duty: World at War.
I have a few problems with Halo that Cod deals with nicely.
1. It takes forever in Halo to kill, so the element of surprise means NOTHING. Additionally, this greatly lowers the value of strategy
2. There is very little cover in Halo, so there is much less strategy and thought involved in succeeding.
3. Some weapons in Halo absolutely dominate in the hands of any player while other weapons tend to suck. In Cod, some weapons are stronger than others, but they greatly depend on play style as to which weapons are your best and the good weapons don't stick out from the crowd as much.
Finally, Cod allows for much more play style customizing with the classes and perks they offer.