Modern warfare 2, Though lacking dedicated servers is still a fun game, IMO I feel modern warfare has better game play, Features, and community, Halo is alright for playing with friends, and forge, but if I was to choose between Halo and Modern warfare I'd pick Modern warfare 2.
most people refer to halo 3 when they say halo enough said halo 3 is like 2 or 3 years before MW2 so putting them up against eachother really doesn't seem fair. If you are saying halo odst then MW2 wins because odst was a major let down