A list of twelve (which is the minimum I'd like to see for a season) with some reasons in no particular order:
1. The Pied Piper of Hamelin 1957 (I'd love to see them riff a full length musical)
2. Road to Revenge 1993 (This would actually work better as a Rifftrax but whoever gets their first is fine)
3. Disco Godfather 1979 (name says it all)
4. Without Warning 1980
5. Quest for the Mighty Sword 1990 ( A kind-of Ator sequel)
6. For Your Height Only 1981 (I can see the crew being HUGE fans of the hero)
7. Death Machines 1976 (this again would be a better Rifftrax one but I'd like to see a lousy martial arts flick on MST3K)
8. Girl Gang 1954 (this crew REALLY needs to riff a juvie delinquent movie)
9. Lightning Bolt 1966 (We need a cheap eurospy movie from this crew)
10. Escape From Galaxy 3 1981
11. Monster From a Prehistoric Planet 1967 or possibly Goliathon 1977 (because we need at least one Japanese Rubber monster movie. Goliathon has a pretty sexy lady who stays minimally clothed to boot)
12. Monster of the Piedra Blancas 1959 (because we need more classic black and white talky scifi)
Bonus:
13: Colossus and the Amazon Queen 1960 (because I left out a sword and sandals film from the season)