Time to get serious - I’ll let keener minds than mine decide on categorie, if any make the final cut:
Brazil (so good, it’s great - Robert DeNiro as and HVAC repairman/terrorist) - this one has to go on the list, even though it is not all that “old”
The Blob (so bad, it’s good and Steve McQueen to boot)
The Man with the X-Ray Eyes (arguably horror - a slippery slope)
The Incredible Shrinking Man (help! help!)
The Fly (the remake - too recent?)
Beginning of the End (unless movies riffed on MST3K are off limits - but it does have Peter Graves AND giant grasshoppers)
Anything with Mothra in it - but preferrably when she is in conflict with Godzilla, Rodan (sic?) and others