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...
This and "Escape From NYC". This is darker and less camp, but has Good Good Potential ! I wish I could go into detail of what could be "W O R K E D" riff wise, but that is not my place & there is no time. So Mike, Kevin, Bill....this is a Classic to me and others form the 80's kids. I know you have a lot to consider.....so if you can see it in your hearts & Heads-that detach from your bodies, grow spider legs, and try to calmly walk away from the scene to Riff this Movie. Much obliged.
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...?