This was one of the first MST3K episodes I ever saw, and it was the one that got me hooked on the show. This is definitely funniest, so funny I actually taped it next time it aired (but have since lost the tape unfortunately).
The movie is a god awful Gamera sequel. Two boys wind up on an alien planet with two brain eating female aliens. Gamera comes to the rescue, but has to face Guiron, a monster with a huge knife shaped nose who makes quick work of Gamera's previous opponent from an earlier sequel, Ghaos, early on. If you look horribly fake looking fights of two sweaty Japanese guys in monster costumes, you'll love this one.
Highlights include the songs "We're going to Ride our Bicycle" and "Gamera is really neat, Gamera is filled with meat, we all love you Gamera".
Unfortunately this episode will no longer be aired because it is a Sandy Frank production. The MST boys made fun of Sandy Frank because of his record for bringing bad Japanese movies to the US, and as a result, Frank refused to allow them to air any of his movies. As a result, this episode was never officially released on DVD or tape, so the only way you can see this is to find a bootleg copy somewhere.