Surprised nobody has mentioned this:
When it's around, Christmas Fiasco. You can either do the entirety of just the Caves portion of the quest, or do what I do: stop after getting the second room where the very first spawn is 4 slimes (splits into 16). I forget the exact number of base slimes, but you can effectively crank out around 40 or so chances at a red slime in about 4-6 minutes (depending on your efficiency/killing power).