If I HAD to name one, it would be Area 51 - I think that the pacing is great, I like the fact that it will throw more enemies at you if you are playing two players.The hidden rounds / "Predator Mode" are nice diversions (Unlike Maximum Force, where the hidden rounds seem to be constantly popping up and often have no correlation to the round you are playing at the time)
But I can't pick one.

My other favorite would have to be Cheyenne. Partly because it was the first lightgun game i played regularly, and partly because I find it hysterical.