BOS4 Alec Baldwin, one of the most indelible one-scene cameos of all time.
You know that character wasn't even in the play, and Mamet wrote it specifically for Baldwin for the film.
The only All-Time quote that has a better backstory is (and you have to imagine James Lipton saying this to introduce DeNiro on ITAS) that the screenplay for
Taxi Driver only had the following stage direction consisting of 7 words: Bickle enters bathroom and looks in mirror.