Can any of these be considered B-movies?
Caveman
Barbarella
Saturn 3
Silent Running
Soylent Green
Logan's Run
Flash Gordon
Krull
Legend
Steel Dawn
Barbarella - more of a cult movie, it's not really poorly done.
But
Krull I would consider a b-movie, because although it's well-done (the spider effects were awesome) the plot puts it firmly in b-movie territory. But... well, then I guess I should reconsider
Barbarella.
Caveman is awesome because as a kid I completely misread the ending; no sadness, just bafflement, LOL!
Flash Gordon I consider to be an homage (to itself?) and not a bad movie at all.
Anyway, what's more important to the designation of "b-movie" -- the nature of the plot ("a robotic John Saxon in a post-apocalyptic world...") or poor execution (bad special effects) or both?
Legend, today, feels silly (plotwise) but the effects have actually held up remarkably well. I think most of those films are cult movies. Some of them might have lurid plots, but all were well-executed.