Who was your favorite NPC in a game to interact with? What made them so interesting?

I had a magic user who at 9th (?) level had the opportunity to join an exclusive mages guild. I passed the test and got in. Lots of colourful characters there, but the one that stood out was the groundkeeper! They had a sign on the grass saying "Keep off the grass" and you just knew that this wasn't a plea to keep the lawn tidy. Stuff would happen if you walked on it. No one had the nerve to do it, so we never found out, but we knew. The other thing about the groundkeeper was that when i first joined the guild, he was introduced by name to me. It was a tricky name to remember, but i managed to get it right from hearing it once for the rest of the campaign. When mentioned to the other players in the party (the name came up every now and then), not one of them could ever remember how to pronounce it. Years we played this game and no-one could ever remember the name but me. The reason? Well, after about six months of this (and it becoming a bit of a running joke in the campaign), the DM said that maybe the reason they couldn't remember it was because they weren't members of the guild...Simple as that. That added to his mystique a bit.

To this day, the people in my group can't remember his name. In respect to him, i won't repeat it here...

Skitterjee and Alormic were a villainous double act in a really fun one-shot a friend of mine ran. They were something like rogue demigods who specialised in warping reality by exploiting holes in language; they were promoting something like textspeak in the bizarre near-future world the game was set in because it was so loose and ill-defined they could manipulate it much more than they could "real" languages; we eventually defeated them by using a cosmic editing device to destroy textspeak.

Anyway, the reason they were so fun was their sense of humour. Witty, slightly sarcastic, they were more like bouncy overenthusiastic geeks with a new toy than cosmic vandals out to wreck the universe. When we destroyed textspeak and blocked them from our world they reacted with a phone call which was half silly banter, half scary threats.