Clearly, Episode 75: If I Could Turn Back Time, I'd Still Cut You In Half is far superior to everything else the fplus has done. I don't exactly know why, but there's just something about that episode that makes it so much better in terms of quality.
In all seriousness though, it's really hard to pick a favorite. I tend to prefer the types of episodes that feature really stupid/incomprehensible people and/or weird fetishes. Hans von Hozel is naturally a favorite, as is Storyprovider. Stand-out moments in general are Portaxx reading the part of the guy who has a Stargate, anything that's read with the "Royalty!" voice and anything Lemon cackles at.
I also have a strange fondness for the sentences "Before you is a gaint of a man" and "The pants Moron wears" from the Reno episode.