Try to provide some deatils of ass kicking
Super heroes out of costume do not count, people using permanent illusion magic do not count (Such as Alucard in Hellsing).
Some of my candidates:
The Beast from Kung fu hustle, a middle aged, pudgy man in a dirty shirt and blue sandals who can head butt through multiple rooms, concrete and a martial artist who uses normal thugs as literal pinballs.
Yoda that Little Green "Jedi baby;)", especially in ROTS When he pwns the uber red guards, or AOTC when the little Munchkin reveals his "Monkey on crack style)
Emperor Palpatine, an old man who needs help and a walking stick to get around, coughs constantly, yet wipes out traitors and their stormtrooper platoons in seconds without blinking.
Dr. Stephen Stranges little Greenwich village house in Marvel NY, which is almost infinitelly sized on the inside and has solo-d and defeated multiple top tiers (Such as Silver Surfer and another character, and summoned people from acroos the galaxy who would be the perfect defense against an invader).
The Fluffy thing that once accompanied Samurai Jack, when He (Jack) gets suckered by the Hill-billy Bounty hunters it gets angry and turns into a mountain sized ferocious spiky killing machine that eats them in seconds, before shrinking back to it's regular fluufy self.
Bruce Banner/Hulk- I know I said no heroes, but he's the classic "Puny on the outside, if Angry rips apart mountains" character.
However, I have another. While not the most dangerous (compared to some mentioned here), he fits the bill perfectly. 
