Wow, now that's a tough question. Lots of great sprites in the game, and to choose one? :O
I am going to cheat and name a mob, weapon AND armour. Or well, clothing at least.
Parrot:
This sprite is well detailed and has really pleasing colours. I love looking at it. Too bad they peck at your eyes in-game, but at least there's a pet that doesn't want to eat your face.
Pygmy Staff:
A simple stick, sure, but the addition of coloured markings and the little hanging skulls easily make it appear to be more than just a stick.
The colouring has not been overdone, keeping most of it brown, so you can easily make it out to be a stick, although the little leaf also helps. I like it a lot.
Duke Fishron's mask:
Very goofy looking, with one of the ears slightly angled and an almost annoyed look on its face. Or is that my imagination?
Also has a very pleasing colour. I'd have one of my characters wear this if I didn't want my characters' faces to be visible.