Favorite Single Letter Character Movie Portrayal?

Which select portrayal of a character, known by a single english letter, in a film or film series is your favorite movie portrayal?

(Poll Notes: Honorifics, ranks and other titles are not considered part of a character's name [Mr. X, Agent X, etc.]. When different people assume the same organizational position over time in a film series [007's M, Q] it creates the illusion of a similar character, when in fact they are distinct options. Multi-film, same actor character portrayals are consolidated into a single answer option intended to span their entire series story arc and represent their full body of work.)

Discuss the topic here.