My favorite NPC might be Nardis. This was a fighter I created as a stock character to keep with me when I ran games. Was he the greatest NPC ever? Nah, not so much. Was he incredibly fun in the games? Yes.
Incredibly, even with face-punchers in the group, Nardis was somehow always the most effective martial character. He was set up to be an effective dual-wielding warrior, and he connected with his targets most of the time.
Nardis also represented an effort on my part to build an NPC specific to pre-made game material. He fought with a one-handed sword, and a star knife. He followed the deity of Desna, and I invoked this in-game. Often I use religion as a way to dress up mood, or add flavor to a game. I rarely foreground it, except in case of clerics, for other characters. Nardis was a product of the world he was created to live in.
He was highly effective. Even more so than the PCs in combat, which at times saved them, but led me to play the NPC a little more sarcastically. The PCs deserved to be teased a bit for being a bit inept in a fight.
Nardis is one of the NPCs I remember the most, and that is a big part of the reason I included him here. Every character has a role to play in every story. Whether it’s an NPC or a PC, everyone can be memorable.