What I mean by "social-situation-bastard NPC" is a character who really frustrated your PCs (or you) through their social actions rather than combat or traps or magic or whatever.

The evil court fop, the obstructionist bureaucrat, the unreasonable priest, the gossip-monger, the 'legitimate' businessman, the reputation-killer, etc.

I could use some inspiration for this weekend.

I had a elven Noble once in a 3.5 game that was  bard. He was seducing the PC's sister and charming (through skill) his mother. Only the PC's knew he was the bad guy and he flaunted it when he could. He eventually was found out but since the players had to break into an elven library and retrieve something from the catacombs below it they could not make waves.

The players were invited guest at his party as he tried to figure out what they knew, hoping they would lead him to the item needed.

after being found out he poisoned them both causing the mother to suffer a stroke which left her partially paralyzed for life.

The elf truly hated that noble. Also the PC was an elven bard as well. It was fun

Second NPC was a Rakasha guy who was a noble in the main city of the campaign. He had stalked and creeped out the cat folk character. They referred to him as creepy cat guy do to his fascination with the cat folk female PC. But he had all the connections. Eventually someone figured out what he was and they attacked is villa during the day rampaging through the household and killing his doppelganger double and eventually getting arrested and exiled from the town.

Same players in a different campaign but same gameworld hated going into that town for fear of running into him.

A long time ago in a Star Wars game far, far away the Players decided it was a good idea to work for the Hutts. So I made their Hutt contact a nasty condescending obnoxious gentry type who thought he was being punished by having to act as go-between with the party. The Players found this was too lucrative to give up, so they put with the NPC.

Until we got a new Player who was a wookiee, the person playing him was this big samoan looking college student so he actually kinda looked like the character. Now after one of these belttling exchanges with the Hutt fop, the party leader  (Retired Imperial Officer) looks at the wookiee and dismissively says, "Get rid of him so we can get to work."

At which point the wookiee broke the Hutt fop's neck. And the Wild Die came up with a 6 twice for the skill roll.

I just sat there for a second, with all I could say being, "Well, that earns a Dark Side Point."

The rest of the campaign was spent trying to stay one step ahead of the Hutt's quest for vengeance against the Players for killing his favorite toadie.

I think my favourite BBEG was a mild-mannered Japanese Salariman corporate exec. For Araska Corporation. The guy wouldn't hurt a fly - in person - he was just trying to maximise shareholder value. I loved it when the PC captured him and realised there was absolutely no point killing him, he'd just be immediately replaced by someone identical.

I partly got the idea from some speculation by Case in Neuromancer on how family-owned Tessier-Ashpool differed from the typical zaibatsus.

Mm ... the NPC I had the most fun with was the Marquis de Bergevin in my main Scarlet Pimpernel campaign.

The character background of the League party's French actress was that she was seduced at 16 by a handsome nobleman out with his buddies in the sticks to raise hell and boff peasant maidens.  Abandoned, pregnant, ostracized, she left her infant daughter in the hands of her aunt and went to Paris to make her way.

Fast forward several years, and Solange's current lover, an up-and-coming Revolutionary assemblyman, tells her that he just came from an interrogation of the captured Marquis, and however improbably, he's asking for her.  How exactly do you know the fellow, cherie?

Up until the meeting, I wasn't sure how I'd play him, and the instant Solange's player said that she was walking into the cell, the thought struck: "Play him wicked."  Whereupon de Bergevin calmly suggests that if she wants to see her daughter again, she'll do well to use her influence to get him out of prison ...

They encountered de Bergevin a number of times subsequently ... in a sword duel with Solange where he mulishly refused to carry the attack or press home potential blows, or later on down the road when he improbably turned into a skilled freedom fighter at the head of a band of guerillas, hissing to Solange, "I know what I am, but before that I am a Frenchman."  

The emotional sparks between the ex-lovers were heightened in so far as Solange's player and I were a new item, and he turned out to be one of the most complex, vivid and baffling NPCs I ever ran.

I had great fun RPing a police officer interviewing the players after a rather strange event in contemporary London.

Lots of leading questions, automatic assumptions based on what they were saying. Really really annoying!

One player got really antsy with me and started shouting "stop acting like a fucking policeman!"

I was just interviewing...

In my ICONS golden-age campaign several of the characters were agents of the OSS; I had King Farraday act as a kind of Amanda-Waller type figure who manipulated politics in the War department to get the previous director of that office out and put himself in; then he got all the PCs out too.  They realize he's profoundly patriotic in what he's doing but they also despise him because he clearly doesn't like mystery men (he doesn't trust them because they can't be easily controlled/managed, in most cases) and he's very much an "ends justify the means" character.  And because he knows how to play the game in both the Intelligence community upper echelons and in Washington, he's outsmarted them at every turn.

RPGPundit

I have mentioned my "Hunter" Cyberpunk games before where the party have been fighting the whole campaign against Lawrence the CEO of Sony Corp. They fight their way into his base kill most of his guys all but one of the PCs are dead. The survivor Dragard D'Bard, former pop iceon turned drug adict turned fixer has been through the wringer he is shot to crap but alive. Lawrence is sitting on the floor in the remains of his smashed glass coffee table the bodies lying round him, gutshot and bleeding out, kind of like the hotel scene in True Romance. D'Bard pulls up his gun to finish the guy that has been his bette noir for 20 - 30 sessions and ... Lawrence overs him a TV show and a lucrative record deal ....

A go-to resource for me is 'Seven Types of Antagonists'.

Unionized Lawful mercenary hirelings.

"I'm sorry, it's in our guild regulations, we don't open doors. All we do is accompany you and fight."

"Sorry, our guild sets our rates. They're non-negotiable."

Drove the murder-hobos nuts.