Note: Yesterday I accidentally entitled Day Thirteen as Day Fourteen. That has been fixed, and here is the content for Day Fourteen of the 30 Day Challenge.
Choosing a specific favorite NPC is tough, especially given all the various types of NPCs across all the games I've played and run, so I'm going to choose an NPC archetype instead: the super villain.
I'm not talking about a comic book super villain like Lex Luthor or Venom. I'm talking about the big bad guys, the boss monster at the end of the dungeon, the major antagonists that players love to hate.
I've already mentioned that I love the dragon's lair as a dungeon location, but a lot of that comes from the fact that I absolutely love smart, evil, dedicated villains as NPCs. Sure, the classic “some orcs in a room guarding a chest” scenario is great; there's a reason it's a classic, after all, but those orcs aren't really interesting unless there's something more there. The orc chief, the hobgoblin warband leader, the dragon leading an army of kobolds, the archmage throwing arcane minions at the party, the slick mayor who's got the town guard in his back pocket: these are villains. They're not just enemies, they're not crossbow fodder, they're villains. The man with the plan.
In my experience, it's the true villains that are interesting, and I mentioned super villains because there's definitely a comparison to be made there. In a comic book, the focus is rarely on the groups of faceless thugs attacking Batman; rather, the focus is, and should be, on the Joker. The Joker is interesting, he's unpredictable, and he's absolutely ruthless. That's what a villain should be: interesting.
Villains are interesting because they provide a counterpoint to the PCs. Where the PCs want to save the townsfolk, the villain wants the town under his control. If the PCs are going after a magic artifact, the villain does what he can to make sure they don't get it. The PCs go after something themselves, while the villain has minions to do his bidding.
That's why villains are so interesting: their sole purpose as a game element is to provide more than “a few orcs in a room guarding a chest.” They give the PCs a reason to adventure: to stop the villain.