Space Mutiny is easily the best “introduction” episode to the show for newcomers and it remains a classic that shows what MST3K is all about.
I do want to give honorable mention to Star Crash, a Jonah-era Italian attempt to do Star Wars featuring a surprisingly famous actor playing the space emperor, a cowboy drawling bounty hunter robot, and a lady wearing transparent plastic that grits her teeth a lot. It’s a good time too.
The Wizards of the Lost Kingdom movies are a fine slice of the fantasy side of cheese, as is the delightfully named Deathstalker and the Warriors From Hell.
Oh, and Delta Knights!
I also recommend, for those that like spy/heist/Italian weirdness, to try Diabolik, and Operation Double-Double-07.
Hard mode might be “Agent From H.A.R.M.” because there’s long stretches of very little happening, but what happens can be downright hilarious.