As the TV season winds down and moves into the May sweeps, the major networks are duking it out for those coveted No. 1 spots, especially in the adults 18-49 group that advertisers love and the ego-boosting title of the most-watched network. This kind of tight primetime competition hasn't been seen in the TV arena in many years, but the reality TV explosion has truly changed the landscape and kept network execs on their toes.