Heaven's Gate: I couldn't even get a quarter way through this monstrosity. It is so slow and boring that I have trouble seeing how anyone could sit through it, much less riff it for three hours (and I love three hours films like The Godfather and Lawrence of Arabia). I won't say it's unriffable (it does have Christopher Walken), but I can't see anyone putting themselves through the agony of watching it for a riff.
A Serbian Film: Self-explanatory.
Supergirl. Actually I think someone could make an excellent riff of this film. That is, if they could ever tolerate sitting through it. I think this is the worst superhero movie ever made. Worse than Batman & Robin, worse than Green Lantern, worse than Fantastic Four. It is so bland, boring and badly-made that it would try anybody's patience.
Anything by Tarkovsky. Try riffing three hours full of slow moving rivers, philosophical musings and shots held for five minutes. Good luck.
My Dinner with Andre: IMHO the gold standard of difficult movies to riff. It's two hours of two guys chatting in a restaurant. That's it, pretty much. Sure, you could make fun of the dialogue. But for how long? I must admit that I did like it a bit because it's so different, but it was also making me sleepy and I had to stop it twice.
I add in any films over four hours, but that's kind of a given. Who could riff Andy Warhol's Sleep or The Cure for Insomnia? Furthermore, who'd want to?