Mitchell and Space Mutiny are probably the best MST3K episodes. They are the two I reach for when introducing someone to MST3k the first time.
They aren’t my favorite, no that is reserved for Santa Claus Conquers the Martians. When I spend most of the mid 70s living at John Hopkins hosptial, it was one of the movies they showed to us as a “treat” around Christmas time. I was 6 at the time and it confused me to no end. When I saw it 20 years later, the MST3k riffing purged the demon from my soul that was Santa Claus Conquers the Martians.
The worst MST3 episode is easily The Starfighters. Using mostly stock footage of fighter planes refueling with some limited ultra low budget single camera scenes to provide plot, it is one of the most mind numbing movies ever created and even MST3K can’t help. No matter how many times the phrase “poopie suit” is said.