This one is interesting because a lot of the time players will forget about NPCs because they are only there for a moment a lot of the time. But in some longer running campaigns they might be there for a very long time. I think my favorite NPC comes from the Rise of the Runelords Module. Lord Foxglove, he has a massive amount of character development over the course of the module. If you so choose as a DM you can even have him escape his death to haunt the party at a later date.