Personally, I'd use a Glabrezu. It perfectly fits the description of "Powerful demon that meddles in mortal affairs covertly."
Then, if you have the Fiend Folio, I'd apply the Fiend of Corruption PRC to it. IIRC, the PRC gives an Alternate Form ability, so you don't have to give it a polymorph item or suchlike. Also it should give you all the social abilities you could want.
Since even a 6th level Glabrezu Fiend of Corruption is no match for a balor, though, if you want it to be that effective in combat, you'll have to apply some other templates or class levels to it. 2 levels of Blackguard should be appropriate. Dark Blessing sends its saving throws through the roof with its high charisma, and Smite Good adds to its attack bonus.
It's up to you if you want to just keep its natural attack routine (The glabrezu's is pretty good) but if not, you could give it a weapon to hold in its claws. Ì suggest a greataxe. A huge greataxe deals something like 3d8 points of damage, and you get the nice 2-for-1 power attack ratio. Plus, you can still make pincer and bite attacks... (A word of advice: The glabrezu has Improved Grab, but unless it has allies who can occupy the other PCs, don't grapple one unless you can crush him in short order. While you're occupied, the others will surround you and chop you to pieces.)
Replace the Confusion power with Charm Monster, so you qualify for Fiend of Corruption, and you're good to go. (Personally, I'd also replace Mirror Image with Displacement, because I find Mirror Image to be a headache of unthinkable proportions. But that's up to you, of course.)
Here's the feats I would suggest: Power Attack, Cleave, Improved Sunder (For Blackguard), Combat Expertise (For keeping the AC up to par), Extra Smiting x2 (To rely on smites, it has to smite more than once per day), Multiattack. If you give it AC-boosting items, or armor, so it doesn't need Combat Expertise, you could give it Quicken Spell-like ability (mirror image/displacement, or dispel magic), to give it a little more presence in the fight. I give it to almost all high-level outsiders, so they don't get out-actioned quite so badly. Or you could look at some of the suggestions below.
Apply the Elite array to the standard glabrezu's scores, as well as the two points for class levels, and you end up with something like this:
Str 35, Dex 12, Con 33, Int 16, Wis 14, Cha 26 (+4 +2 +2 +0 -2 +6)
If you need an even tougher opponent, add more Blackguard levels, or levels in Fiend of Blasphemy (also from Fiend Folio). And remember to give it CR-appropriate gear. (I tend to use gear as an NPC of its CR, but that's up to you of course)
Technically, if you add more than 8 levels (6 fiend of corruption, 2 blackguard) it gets 21 HD and is eligible for epic feats. That might be too nasty for your PCs, but if not, I suggest Great Smiting. Good non-epic feats include Combat Reflexes, Improved Initiative, Empower Spell-Like ability (Unholy blight), Improved Natural Attack (pincer), or Combat Brute (From Complete Warrior, that one's evil)
If you don't have Complete Warrior, you can find Combat Brute here:
http://realmshelps.dandello.net/cgi-bin/feats.pl?Combat_Brute,CW
There's a bunch of other feats on that page, too.
Good luck!
