Russo-Finnish/German/Mexican Fairy Tale Movies
Some of my favorite MST3K episodes were the Russo-Finnish fairy tale movies--which even the guys at Best Brains admitted were beautifully filmed. Jack Frost/Father Frost was probably my favorite of the Sci-Fi channel era.
There are loads of other Russo-Finnish fairy tale movies. Many of them have been released on DVD through the company Ruscico; they can also be found on YouTube in their original language (most of them subtitled in English). It would be great for Rifftrax to cover them (or re-cover the ones already done on MST3K), but for the ones you haven't covered yet, it would depend on whether they'd had official English dubs. (I know of some of them which have, such as Through Fire, Water, and Brass Pipes, The Snow Maiden and The Princess And The Pea. I believe the award-winning The Stone Flower may have as well.)
Also, Germany, in the 1950s, made several live-action fairy tale movies that were dubbed into English and released on the kiddie-matinee circuit. Snow White, Sleeping Beauty and Rumplestiltskin were among them. A few of them were shown on the BBC--most notably, The Singing Ringing Tree, which frightened a generation of British kids. (The latter was unusual in that it wasn't so much dubbed as voiced-over: we can hear the original German dialogue while a narrator translates it and tells us what's going on.)
Finally, there have been some fairy-tale movies made in Mexico and released with English dubs--mainly, the Little Red Riding Hood trilogy, released on the kiddie-matinee circuit by our old pal K. Gordon Murray. These three movies (starting off with an adaptation of the original fairy tale and following up with two original stories) feature a couple of familiar faces: the wolf and skunk characters that showed up in the Santa's Village of Madness shorts, also cobbled together by K. Gordon Murray. The third one features Jose Elias Moreno as the Ogre--he played Santa Claus in the infamous Mexican movie. They are all musicals, and the first one features, I kid you not, the skunk serenading a parrot. A fan-made continuation of MST3K covered the first Red Riding Hood movie: check it out at the link featured on http://www.mft3k.com/episodes/F01/index.html. Perhaps you guys could outdo it...
All of these would be great riffing candidates for the same reason the first four Russo-Finnish MST3Ks were: they are often beautifully filmed, with some great visuals and lovely musical scores, but have a fair amount of utter insanity going on as well.
Charles Hodder commented
Some of my favorites as well.