Book of Vile Darkness: Demonologist

Today's excerpt has garnered some negative criticism partially for the outfit, but largely because of the quasit's anger issues; most of the time it gets along just fine, but if you are unable to give it orders--like you are stunned, dominated, or sleeping--then it "acts under the Dungeon Master's control as your enemy".

Despite this drawback it is still a pretty sweet package; adjacent enemies take a save penalty, it doubles up on ability score damage (determined by you, no less), can turn invisible whenever it wants to, can grant you a skill bonus, and can even save-ends daze a target that attacks you. Since it can communicate via telepathy, you can have it scout places while invisible and get some excellent intelligence because it has to do what you say. 

And that is just the start. You still get your action point benefit--adjacent creatures take variable energy damage and you gain resistance for the encounter against that type--and an encounter attack that lets you bind a demonic essence to a creature, which lets you slide a target, have it make an attack with combat advantage and a damage roll. If the attack hits? The attacker is then also dazed for a turn. Very nice.

At 12th-level you can levy an evil-eye type effect against an enemy as an interrupt, imposing an attack penalty and causes them to grant combat advantage for a turn. It is an effect, so no attack roll required.

The level 16 feature causes your quasit and all summons to gain a damage bonus, so there is extra incentive to take summoning magic (not that I needed more).

Finally at 20th-level you can summon a huge blast of hordlings that deals damage and prones targets. It also creates a zone that lets you spend a standard action to deal automatic damage and prone all creatures inside (or more damage if they are already prone). Oh, the zone lasts the entire encounter. The only downside is that it is not friendly, so you gotta be careful not to tear up your allies.

I like this paragon path because it is very thematic. I do not really give a fuck if the quasit can be abused by douchebag DMs, as you can always kill it and summon it later anyway (I would just allow a character to dismiss it). Otherwise I can see it bringing some interesting social roleplaying to the table, giving suggestions and trying to tempt the character. I would have honestly preferred it to be an imp (devil), because then it could literally play devil's advocate.