That's easy: it's Cosmic, of course. For proof, look no further than the world-famous Trashdome, where two games enter ... but only one game leaves!
And, this time, the last game standing was Cosmic Encounter. All hail the champion!
(Warning: Fortress Ameritrash often features some ... uh ... "colorful" language and intense debate. If a few F-bombs are enough to make you uncomfortable, you might want to shield your eyes)