The Thing (1982)
Considered a cult classic, this sci-fi horror film from John Carpenter is his best, but would fit right in to the Rifftrax catalog. It may have to be a "Just the Jokes," but I feel like all the blank spots in dialogue would make this a great riff.
Stars Kurt Russell, WIlford Brimley, Keith David, and more...
I've given this a vote because a) there are quite a bit of silent breaks in dialogue that could be riffed, as you stated, and b) favorite horror film of all time. However, it may be too squishy and gross for the gents, but...they did do Treasure of the Amazon & the Dark Power...maybe they'd do some Fulci...?