1- Harry Greb

2- Henry Armstrong

3- Aaron Pryor

4- Stanley Ketchel

5- Rocky Marciano

6- Khaosai Galaxy

7- Joe Frazier

8- Mike Tyson

9- Jack Dempsey

10- Beau Jack

These fighters were relentless in your face type of fighters. Honorable mention: Roberto Duran and Wilfredo Gomez.

Roberto Duran

Mike Tyson

Harry Greb

Mickey Walker

If you meant "biggest in you face nightmare" as "intimidating".

Hands down, Mike Tyson, muscular physique, malevolent glare and a ring costume of nothing but black trunks and shoes, he looked the part of a homicidal henchman from some dark lair. Proof? Tyson’s 94 second demolition of a terrified Michael Spinks; Frank Bruno walking to the ring as if it were the gallows; or consider that more than half of Tyson’s 44 career knockouts took place in the first round.

Honorable mentions: Jack Dempsey, Robert Duran.

Good question with different variables that can give different possible answers, here's my list off the top of my head-

1) Harry Greb

2) Henry Homicide /Hank Armstrong

3) Gene Fulmer

4) Carmen Basilio

5) Rocky Marciano

Americos Santos ,Antonio Margarito and Paul Williams are 3 that comes to my mind.

Hello my friend, I can't top Tyler Durden's choices but I will add Roberto Duran to the list.