Menace From Outer Space (Rocky Jones: Space Ranger III)
'Manhunt in Space' and 'Crash of the Moons' were two of my favorite MST3K episodes, but there are still plenty of Rocky Jones "movies" left to riff. I'm not sure of the availability of later episodes, but 'The Gypsy Moon' (featuring more John Banner!) and 'Menace from Outerspace' were both released independently on DVD, and as far as I know, are now in the public domain.
You can watch the entire "movie" here.