For me, it's Ninja Gaiden I and II. They're not [i]great [/i]games, but sometimes they're exactly what I need. 
Underrated means non-mainstream or not highly praised.
Go Floodies.
						
					
					
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	Aliens Colonial Marines. While the campaign was [b]certainly[/b] underwhelming, the multiplayer was quite entertaining and is definitely worth a play.