My personal preference would have to be the Call of Duty franchise, personally, preferably Cod4, followed by Black Ops.
Halo is a good, solid game which does have some good and intuitive Game modes. However, the one thing I don't personally like is the amount of health on the game; I feel like i'm cheated out of kills sometimes if they get a well placed headshot, or a one shot from a shotty. This means I have to play Team SWAT most of the time to feel comfortable.
The health on Call of Duty means you have to rely on fast reactions, precise aim but most importantly, being faster than your opponents [unless you camp, which I don't] [which is also why I use max sensitivity], which in my opinion, makes the battles on CoD feel more intense and rewarding.
However, this is just preference; there is no better or worse game. It's purely down to opinion.