I really want to see them tackle The Godfather. Mike has mentioned it several times before. And this movie, along with LOTR and Raiders indicate that it could happen eventualy.
The only movies that are truly unriffable would be a genuinely funny comedy (and even a bad one like Catalina caper is a bit harder than the usual MST3K), or a tragic movie like Schindler's List where the riffers would look like absolute tools.