So I have used a Devil Prince as an NPC in a couple campaigns, he is always the mastermind behind the scenes, but the party knows he is there. He normally shows up early and the PCs are tricked into making a deal with him, normally they get out of it because he sells the debt to a good NPC (makes sense in the story) and manipulates her and the PCs into doing work for him without them knowing (he shows up clearly profiting from their jobs, or clearly moving his plan forward)
He has a spy they know too well too, she's a Rakshasa Sorcerer, very manipulative herself and can change her shape and stuff so she is very good at disguising,
I know they are going to get to the point where someone gets angry enough (and confident enough) to take him on, but I have not stated him out yet. I'm looking for class suggestions. He's a high level, so multiclassing or prestige classes are do able. He is susposed to be very charismatic, extremely intelligent, slender built (not strength based) magically inclined, and avoids combat at almost any cost. He uses diplomacy, intimidation and manipulation and bribery to get what he wants.
Sorcerer and Bard jump out as charisma-based spellcasters with access to the types of enchantment spells that such a NPC would find appealing. Sorcerer has the upshot of having a much larger magical arsenal, but Bard gets more skill points and class skills so I could see going either way. I'm sure you could make up the difference with a decent intelligence modifier, so it really comes down to what you want to do with it.
If you want the character to be as annoying in combat as he is outside of it, my suggestion is stack on effects that make it very hard for the party to actually see what's going on. Have the villain fill the battlefield with fog, darkness, pits, black tentacles, and any other nasty obstacle you can think of. He does all this while under the effect of Greater Invisibility, of course, so even if the party has countermeasures to any of these individually it's still very hard to target the villain. You can use the Echolocation spell so the villain can see through his own fog and darkness.
I was thinking bard or sorcerer as well.
However, I thought you wanted an annoying PERSONALITY, rather than annoying combat. I was thinking over-confident, self-entitled, and possessing pre-planned escape routes for a recurring role in the campaign. If they can point out obvious character flaws or mistakes the PCs have made, then even more annoying.
If you meant annoying to combat, then illusion spells can be very annoying. Illusionary doubles, disguised hazards, invisibility, etc. But keep a teleport handy for a quick escape.
Thanks everyone, really like those suggestions!
Im surprised that nobody has suggested the Mesmerist yet, this is almost EXACTLY their niche