So I was thinking about it, which fictional character's do you absolutely, do not want to fuck with. The type of character that is really just a force of nature that wins, period. Now granted it will come down to the author's writing style, as they are the creators of these character. This is to go across all medians, books, comics, tv, movies, video games. My one rule/request would be that you only pick 1 character per series, and please, specify where the character comes from and make your case, do not just list characters.
1: Mitch Rapp - The Mitch Rapp series by VInce Flynn - He is an experienced CIA counter terrorism specialist, in other words he kills terrorists. He is physical freak with a close to genius IQ whose only goal is to protect the United States from terrorism. His name alone instills fear in this enemies and can cause some of them to quit the game.
2: The Doctor - Doctor Who series - More specifically the 10th Doctor. Another that does want is necessary to win and is usually on the right side. Again, The Doctor is another character whose mere name will terrify his enemies and force them to reconsider their actions.
3: Minerva McGonagall - The Harry Potter Series - I know she kind of seems like ad odd choice here, and maybe I picked her partially in the hope of sparking more debate, but she certainly does fall into this category. Dumbelore might be more powerful and Voldemort more evil, but McGonagall is plenty powerful and not a witch to be crossed. I list her her because I believe that her personality more so then that of Dumbeldore's lends her to this category more then him.
Honorable mentions: Batman, Jack Ryan, Dumbeldore, Tywin Lannister, Percy Jackson, Snape, Aragorn.