For me personally, it's the Season 8 episode "Space Mutiny", which was also the first MST3K episode that I ever watched.
That bit with Mike bonking his head on one of the many railings Servo placed in the Satellite of Love and tumbling all the way down it while yelling before splashing into water sent me into an absolute laughing fit. I laughed for a good 10 minutes at that, I kept replaying that scene. It was pure comedic gold.
I totally agree! The best introduction for first-time watchers of MST3K. It really is the best of MST3K's riffing from Season 8, and in my opinion, let alone the whole series. I am glad I picked Space Mutiny as my first episode. I got a great introduction to the series, and I have been a MSTie ever since. If you haven't seen Mystery Science Theater 3000 yet, start with Space Mutiny.
You will love it.