Tough to pick a favorite. Maybe the one where they rip into the Nostalgia Critic? Maybe Super Bummerman.
As for just riffing on games, either Earnest Goes to Anime or the Jurassic Park one, as both are bats*** insane and the guys do a great job going along with the crazy, crazy ride.
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Current Let's Play: Final Fantasy IV (FF2US)
http://forums.somethingawful.com/showthread.php?threadid=3445328