I find slimes really cool because unlike most monsters they are a very recent invention, iirc the first slime monster is from a 60s movie, and they aren't as settled in as other ones, so you can get a lot of interpretations and version without much problem (while keeping some common parts, like shapeshifting).

Slimes with unusual materials are cool (like lava, honey, water, liquid metal...) and it's always nice to see them, what are some you like, and what abilities do you give them?