Another route might be to figure out which movies people likely already have in their collections. Raise your hands, folks, if you have one or more of the following movies in your personal collection:
Speed
Total Recall
Eraser
Tremors
Stargate
Aliens
Terminator 2
Gone In 60 Seconds
The Italian Job
National Treasure (1 or 2)
Any Stephen Segal Movie
Any Jean Claude Van Damme Movie
Ladyhawke
Days of Thunder
Rocky IV
Most can be found for a bargain, but let's be honest, most movie fans have these movies sitting, collecting dust, on their shelves anyway.
I've got Aliens and Tremors. Not sure either would be a great riff choice (Tremors especially, since it's already successfully humorous.)
I like riffs of good movies and bad ones fairly equally. Then again, I don't see riffing as an insult as much as a roast. We kid because we love kind of thing.
I prefer riffs of bad movies, to be honest... Even if every riff joke isn't a dig at the film, in general it it all feels like the riffing is making fun of the film. And it just feels wrong when they're making fun of something that's good. It's like listening to a comedian with a false premise. "How come on airplanes, they always serve giant, roast chickens? How are you supposed to cut it with those tiny plastic knives, and on the little tables and" and you're thinking, "Uhh... I fly all the time, and they never serve giant roast chickens." So you're just not on board (sorry for the pun) with the humor. Plus, on bad movies, you know it doesn't matter when the riffers speak over the dialogue of the film, because it's all expendable crap. But on a good movie, you want the riffers to shut up so you can hear the great speech, or to stop breaking the flow of a really engaging scene.