My vote for the best "Really Pretentious Game" on Xbox 360 is Jonathan Blow's Braid.  Through its beautiful art style and innovative use of time-based play mechanics, Braid does something any good pretentious game should do: Show us something we thought we understood, and reveal it to be deep and intelligent in a way that mocks our meager expectations, in effect undermining our very existence. 

Look closely.  Our protagonist is named Tim.  A normal, ho-hum everyman?  So you think.  Now take that name and flip it around--rewind it, so to speak--and what do you get?  M-I-T.  Only a genius wunderkind enrolled at the Massachusetts Institute of Technology could appreciate the subtle in-joke on display here.  Man has no hope for survival in the future.  One day soon, computers will become our everyday heros; in time, the princess will have no plumber to save her, only a complicated series of zeroes and ones. 

You think you're so cool, Braid.  Well I say keep your brilliant level design and atmospheric soundtrack!  I've had enough of you flaunting your better-than-me charms.  If you need me, I'll be making a WarioWare D.I.Y. microgame where Tim ages a decade with every tap until he dies.