Rainbow Six is a much more complex, and difficult game than Gears. I personally don't like Halo because it's pretty much average in every aspect, and it's too sci fi for me.And again, Gears of War is a third person shooter, and should not even be in this conversation.
Hell, IMO, CoD is a more difficult game than Halo. It also has a much better online community, which is another huge drawback to Halo. I can't stand playing Halo online and listening to all the racist, screaming, idiotic people playing that shit.