The ones I love offhand:
The Wizard of Oz- I was worried about this, since I love the classic so much, but it was great, IMO.
Santa and the Ice Cream Bunny- a glorious pinnacle of madness whether the live version, studio version, or both!
Fun in Balloonland- technically a short, but a long one and my absolute favorite Rifftrax thing ever.
Speaking of which, any/all of the short compilations are fun and would be my top choice for showing to people new to Rifftrax.
Jack the Giant Killer- I think it’s only the live version, but I could be wrong. It’s fun, whimsical, and hilarious.
Attack of the Super Monsters- I’ve clamored for awhile about how much I’d love folks to riff animated features and this is the first one I’ve seen done by MST folk. Plus, rubber suit kaiju!
Plan 9 From Outer Space- an absolute classic and one of their earliest, so perhaps best for a MST fan to dip their toe into Rifftrax… also, any Rifftrax version of an MSTied movie could be fun for comparison/contrast-- Manos, Santa Claus, Santa Claus Conquers the Martians, etc.
Birdemic Live- less problematic than the studio one, if I recall, but still gets a bit weird with the fat-shaming of the mom… still has some great gags and terrible, terrible effects
Ghosthouse- a great one with some killer jokes
The Star Wars Holiday Special- I love the inclusion of the commercials and it’s so lampoonable!