There are some movies of his that are watchable. I recommend his Poe movies, The Terror, and Dementia 13. Don’t expect his Poe movies to match the books. His work with Boris Karloff makes it weird to watch How the Grinch Stole Christmas.
As far as actual GOOD Corman movies, check out his 1964 movie The Secret Invasion. It’s kind of a proto-Dirty Dozen type movie. A couple scenes could’ve used another take, but for the most part it’s really well done. Shot on location in Yugoslavia, it stars Stewart Granger, Raf Vallone, and Mickey Rooney. But Henry Silva steals the movie, doing some of the best work of his career. Highly recommended!
My favorite of the films he directed are probably A Bucket of Blood (1959,) a dark comedy starring Dick Miller and Barboura Morris , and Attack of the Crab Monsters (1957) with Richard Garland and Pamela Duncan.
Regarding his MST3K titles, I’ve always thought that It Conquered the World and The Undead were both good in their own right. I suspect the reason Corman gets ragged on is because he’s not an arteest. However, he’s something just as important. He’s a professional, able to get a movie in the can on budget and on time.
Throw Gunslinger in there, too. Something that all those movies share are strong female characters who realize they have agency and then act on it. It’s Beverly Garland who confronts Carrot Top in It Conquered The World; Peter Graves only shows up in time to offer the (excellent) soliloquy.
Corman’s THE INTRUDER, THE ST. VALENTINE’S DAY MASSACRE, FRANKENSTEIN UNBOUND, his POE films, and the work that he produced for Jack Nicholson and Peter Fonda, Peter Bogdanovich, as well as the Ramones ROCK N’ ROLL HIGH SCHOOL, and many others he oversaw were fine. He could be very inventive. Some scenes he directed were colorful and surreal, using gels, mirrors, filters, for dreamlike effects. His production was like a guerilla film school, and Joe Dante, Ron Howard, Jonathan Demme, James Cameron, and John Sayles were mentored by him. Lots o’ cheese? Yeah. But some nice work. His interviews are always fascination and well spoke. I loved his turn as a host for AMC’s MONSTERFEST, when it was fun and not slasher movies.
The Intruder (mentioned by @Some-who-call-me-Tim) is sometimes considered his best film. It was a “serious” message movie and it was his first movie to lose money. And he said to himself, “OK, my job is to entertain, first and foremost. Any message is secondary.” (According to Joe Bob Briggs, The Intruder finally went into the black in 2014!)
I admire the humility of this mindset.
His best directorial work has been mentioned already, I’d say. (I, too, am a fan of the black comedies Bucket of Blood and Little Shop of Horrors.)
As a producer, he was one of the first to realize foreign art films could make money in the USA, and was responsible for Breaker Morant, The Tin Drum, Fitzcarraldo, Shogun Assassin and many others getting time in cinemas back in the '70s.
Timely, as I was recently thinking about how Roger Corman sits at the center of a vast web of connected talent. Possibly moreso than any other person in film (even Kevin Bacon). The number of actors, directors, musicians, and other talents who worked with him at one time or another is absolutely astounding.