This is a great topic
Surprised to see so many Freezeezy Peak lovers here, was never a huge fan because of the slippery snow!
My favourite is tied between Mumbos Mountain and Mad Monster Mansion, love Click Clock Wood too though.
Rusty Bucket Bay is the bane of my life when completing this game! Feels too small, not enough fun and too many things that can kill you! I second the person above with Clankers Cavern though, I've played countless times over a decade and I nearly always die when freeing Clanker!