Kroot & Krootling! My favourite part of the whole Tau empire; which in my opinion should have been Star Treks Federation rather than a weird mash of Gundam and Red China tropes, but I digress.
I like their eating/absorbtion based powers and how they're kind of just chill as far as beings go in the 40k 'verse.
I'd like to play as Big Dad Kroot in a 40k RPG someday.