There have been plenty of NPCs that we have crossed paths with over the past 30+ years. Some have been downright dangerous - assassins in a variety of shapes and forms, wizards, a particularly nasty and persistent vampire, and a couple of anti-paladins for good measure. Others have been "local flavor" varieties - nobles, innkeepers, Jimmy the Houseboy, and grizzled old men who all spoke in a nearly identical crusty, haggard voice (Ned is the bestest DM ever). Each one has in one way or another added depth, humor, danger, or suspense in our many adventures.
As far as memorable NPCs from TSR, I have to go with Icar the blind fighter from Secret of the Slavers Stockade as my favorite:
"Icar is second in command to Markessa the director of this slaver operation, and a battle trained veteran. Icar is blind and has been from birth. But special training he received while a child at a monastery, has made him remarkably sensitive to sounds, air pressure variations and smell, and he is thus able to "see" better than a sighted man."
There is just something very cool and very unique about a "7' tall, black man in black plate mail armor" with a +1 two handed sword called Death's Master. Icar is immune to illusions and visual spells, but susceptible to silence spells. Thanks to a ring of fire resistance, he is also able to stand in the firepit and throw handfuls of flaming grease at the party (see the very cool Jeff Dee picture on the front cover of the A2 booklet).
Best use of NPCs in an adventure has got to be The Village of Hommlet. And I'm not just talking about Rufus and Burne (His Most Worshipful Mage of Hommlet) and Lareth the Beautiful. Almost three dozen structures are populated with NPCs, including my favorite bar: The Inn of the Welcome Wench.
Honorable Mention goes to Eclavdra, the 10th level drow cleric/fighter from Hall of the Fire Giant King. Her name is brought up several times in the series, and it just sounds exotic and dangerous.