Castle of Fu Manchu has pride of place as my least favorite MST3k episode, at least, because the riffs can't save it from being mind-numbingly boring. Manos, on the other hand, winds up completely hilarious.

I haven't seen Legacy of Blood; the Cinematic Titanic I saw was War of the Insects (onscreen title: Genocide), and was a nonsensical and disjointed piece of crap (with a totally gratuitous downer ending), but I suspect it's not as terrible a film. At the very least it's less disgusting.