I think it's a three way tie here.

1. Mike ("Miche") Zacharias (Attack On Titan)

Mike is the quiet soldier in the back. He's rarely shown throughout the manga or anime. It's too bad, because his skills are second best to Levi Ackerman's, who gets plenty of screen time.

2. Ryuk (Death Note)

(Credit to the artist. Comment if you know them!)

Crafty Ryuk got bored one day and decided to have fun. So he dropped his Death Note, a book that kills any human when their name is written in it. Ryuk is along for the ride in the twisted show, cackling every step of the way.

3. Juuzou Suzuya (Tokyo Ghoul)

It's hard to hate a kid. It's hard to hate this kid unless you're his enemy. With a troubled past, Juuzou is hilarious and badass, at the same time still acting like a child. His character is one of the most well-developed in the show.

Honourable Mention (this was hard to chose):

Edward Elric (Fullmetal Alchemist) [Had to do the nendroid toy because it was so funny]

Lust (Fullmetal Alchemist)

Nile Dok (Attack on Titan)

(Credit to fellow-traveler)