My favourite "non-mainstream" languages are, depending on my mood at the time, either:

- Array languages such as K or Uiua. Why they're good for AoC: Great for showing off, no-one else can read your solution (including yourself a few days later), good for earlier days that might not feel as challenging

- Raw-dogging it by creating a Game Boy ROM in ASM (for the Game Boy's 'Z80-ish' Sharp LR35902). Why it's good for AoC: All of the above, you've got too much free time on your hands

Just kidding, I use Clojure or Python, and you can pry itertools from my cold, dead hands.