What kinds of master villains make for the best opponents in Black Crusade?

I like the stats given for the Necron Lord (ToF) for when I want a badass that's served by legions of lessser badasses.

Only War has given me an Ork Warboss and a Dark Eldar Archon (and Succubus and Haemonculus) and these can be fun too in very different ways.

I'm not particularly fond of the greater daemons. They just don't seem as interesting to me, and that's worrisome since this is a Chaos game.

So, what master villains do you enjoy using as a GM? Which ones do you enjoy facing as a player?

As a GM, I tend to heavily favour humans, because I enjoy making "whole" characters with flaws, hopes and ambitions, and Xenos just don't allow you to do that in the same way, as they all have some trait that trump their personality (orkish love of fighting, eldar selflessness and will to survive as a species, necron logic and lack of emotion, etc...). Which is not to say they don't play their part, but one that's usually more contrite and to the point. I like that they remain alien to the players.

However, daemons, being mainly embodiments of emotions, ideals and flaws, make good pseudo-humans with more aetheric goals, that don't HAVE to make sense. All daemons of Khorne need not be all like "KILL! MAIM! BURN!" all the time, and so goes for all the other kinds.

While I agree that the xenos are usually presented in a limited rage of personalities and I do tend to use human (more or less) adversaries as my primary antagonists, I'm looking for something different right now.

I was under the impression that Necron Lords and Crypteks (possibly also Lychguard and Triarch Praetorians) have full emotional ranges - their only limitations are total hardwired loyalty to their superiors (but not necessarily any such feelings towards their equals). I like to keep these guys for background mysteries (along with Dark Eldar) and endgame threats (in preference to the Tyranids, which I really don't like).

For frenemies/rivals, I also really like the Blood Axes where Orks are concerned. They are more human-like than the typical Ork (usually a Goff stereotype) and that can actually make them more relatable.

Dark Eldar are a lot of fun for me. I can always tailor them for whatever power level is needed. Whether a lone Sybarite and his band of warriors or something up to a full Kabal, I can always have something that challenges the PCs' warband. They've been written into so many products for the various lines that I can always find variety with them.

Perhaps Tau? I have a certain fondness for the high tech space communists myself.

They give good adventrue seeds and some bloody hard fights. The warband could be trying to retrieve an artifact on a world the Tau are colonizing or the Tau could want it themselves for research purposes. The Tau being so naive could quite easily be duped into willingly being a sacrifice or even accepting a Heretic/Marine who will then betray them or steal something important.

Although it's not quite a villain they are some very tough adversaries. An Ethereal as the mastermind behind them trying to convert/conquer a planet, maybe one belonging to the PC's. Having to fight a Battlesuit Commander who's leading the force or even specifically after the PC's after they sacrificed a Tau colony settlement for a ritual. Stuff like that. And as the Tau don't like static fighting it gives the PC's more of challenge running battles, having to deal with hit and run attacks. Hordes of Drones swooping down.

For actual Villains Demon Princes are quite good. The badassness of a demon with a lot of the human motives and emotions. As a general rival or sending the PC's on missions to find stuff just to back stab them. I've always liked the sheer brilliance and stupidity of sending people of to find the things that create the item your weak against, just so you have it and hope they don't figure it out and use it against you.

Most of my master villains have been human, (or near-human). I give them an agenda that is seemingly far disconnected from the themes of the main plot line of the campaign. I have very bright players (or I'm not as clever as I think I am?) and they usually figure things out, though that's often when the hip waders are needed.

Perhaps you could use an unbound Daemonhost?

Hm. There is a Daemonhost that "broke" it bonds...in what supplement was that? It was intended as a repository for forbidden knowledge...? Ah, here it is: Kapeth-Shem (pg 30 of Daemon Hunter), but it is imprisoned once again.

Or T'Zar the Broker, Herald of Tzeentch (pg 136 of The Navis Primer), using Mask of Deceit (pg 54 of The Tome of Fate).