Last time, the least powerful devil was covered. This time, it's one of the strongest (the Paeliryon has a higher CR, but it looks like an overweight draconic dominatrix).
Let's start with the ability scores, which are stellar (as one would expect). A pit fiend's lowest score is 26, its highest 37. Natural armor is in the mid-twenties, but it is less important at these levels. Large size is as good as it gets for a PHB devil, and the natural attacks are great, with most of them remaining usable even when wielding a weapon.
Constrict and Improved Grab would be worth it if anything would still be grappleable at this level. Devil Chills doesn't just have zero usefulness, it loops back on itself and creates a closed loop of negative utility. The fear aura is free with a decent range, so at least it'll be useful in some fights. Poison is only remarkable for its secondary damage (death) but if one wants to kill stuff with poison they're a dozen levels late. Regeneration, as has been said many times before, is nearly impossible to penetrate.
A pit fiend's summoning ability is equaled by no other devil. No chance of failure, two uses per day and a very extensive list ensure at least half their battles will be joined by an ice devil, horned devil, or erinyes.
Finally, there's the SLA's, which are very hit-or-miss. Blasphemy at-will seems strong, but there won't be many creatures with under 19 HD to use it on. Create Undead is either a personal mohrgpocalypse or an uncontrollable wild-running mohrgpocalypse, depending on whether the party has a cleric or necromancer. Fireball is far too weak at these levels, but GDM and Greater Teleport remain useful. The other SLA's range from weak (Invisibility) to useful (Persistent Image, Mass Hold Monster).
Worth mentioning is the free Wish 1/year. However, unless one starts implementing fast time-cheese to shorten the recharge time, I don't think it warrants balancing.
So what LA is the pit fiend? Compared to the balor, it's got better strength and natural attacks, but the balor has better SLA's and summoning. I think an equal ECL is about right, which translates as +2 LA.
All are welcome to dispute this, as always.
FINALLY! I thought the devil section would go on forever.