I still vote that the 60s trash movies they riffed are far worse than the roadhouse/drive-in shlock like Manos. If I had to pick the absolute worst film they did, so painful it was hard to riff, by far it goes to Incredibly Strange Creatures who Stopped Living and Became Mixed-Up Zombies. I at least understood the 'plot' of Manos.
One would assume that the worse the movie is, the easier it would be to riff, but the 60s trash movies it is often unclear what the fuck is happening on screen, lines are often hard to understand due to incompetent film making, etc its a wonder MST3k even bothered.
Yeah, In many ways, Manos is the ideal MST3K movie. It has enough of a plot to where you can follow it, and its inept in ways that make it hilarious. It has a unique set up that makes it stand out over other films, its unique in its poor craftsmanship in a way that not many other films are, and it has memorable characters like Torgo and The Master.
For me, if I was in the movie selection process, my standard would be 1) It has to have a story that I can follow with a beginning, middle, and end and 2) It has to be memorable on its own without the riffing, and there is more flexibility with that one as there are many different ways to be memorable, whether that be through its characters, its dialogue, its sheer ineptitude, or its insanity.
Say what you will about the Netflix seasons, but I felt like the movie selection was about as top notch as could be expected.
While the 50s/60s provided some great stuff for them (GAMERA!), it also gave us some drab and terrible films too that are unwatchable even while being riffed. Monster-A-Go-Go is the one that stands out to me the most as just being an incomprehensible mess.
One of the reasons I kind of lean heavily towards favoring the Mike years is that they went for pure colorful and wacky trash. Hobgoblins, The Final Sacrifice, Space Mutiny, Werewolf, Soul Taker, Future War, and so many others are not only more modern, but they are so ridiculous that they make perfect fodder for riffing. It just seemed like a better collection of trash for them to riff on, and it worked wonderfully.