Because 3.5's demon portraits are always feature multiple demons (for some reason, giving fungi a solo picture is more important than giving balors one), I'll be using the pathfinder ones.
Babaus: demonic spies and assassins covered in acidic slime. Their 7 RHD are outsider type, which is pure gold for a roguish character, and their ability scores are high to above-average. The only disappointment is a relatively low dexterity, but when you're as strong as an ogre and have skin like full plate, do you need dexterity?
Babaus have a variety of abilities. They have DR that is moderately difficult to overcome, a few resistances or immunities, low SR that doesn't scale (don't bother with it, I'd say), slime that damages whatever touches their skin (a dagger will probably be gone after a single hit, a longsword after two) and telepathy.
Then there's the major abilities. Sneak Attack is good to have and qualifies them for a few prestige classes (note that Craven won't work with it, though), summoning another babau can be useful in a pinch, and the spell-like abilities are just icing.
How to advance a babau? Dips in rogue and assassin upgrade your sneak attack to 4d6 at the cost of two levels, and from there you can add levels in either of those classes. Their high charisma may make gishes an attractive prospect, but I don't recommend it: you lack the levels required to get a good spellcasting base set up.
All things considered, babaus are strong monsters, albeit held back by their lack of focus. Rogues outdamage them, wizards outcast them, and warblades outmelee them. Still, they deserve a LA, which I'll put at +1 for now.
Feedback is appreciated!