The Bride of Slushhead is one of my top favorites. Even though it wouldn't make sense continuity-wise, this wedding episode should've been saved for the very last one. Too funny seeing the mutants all dressed up in tuxedos. Staghorn ruining the cake by jumping out of it. Oh and there's a part where Gepple (one of the scientists) gets slapped across the face and says something like: "WHY did you do that?!"
"I saw it in a movie."
Gepple: "This isn't a movie! This is a cartoon!"
Four Ways To Sundown and Final Invasion are also in the Top 5 for me.