The balor: strongest of all MM demons. Also a pain to balance for player use because it's part of the variable and misty realm called 'post-epic', where all attempts at a balanced game are thrown out of the window.
Balors have 20 outsider RHD, which is a bit of a double-edged sword. Speaking of double-edged swords, they get one! It's not entirely sure how the ability works, but a liberal reading would imply that you have a +1 vorpal sword with you all the time.
Then there's a ton of other abilities I'm not going to evaluate one by one. A few highlights are the spell-likes (At-will Blasphemy, Dominate Monster, or Power Word Stun? Yes please), your wonderful ability scores (lowest are dex and int at 24-25), common energy immunities, continuous undispellable True Seeing (screw that, entire school of illusion!) and ability to summon another balor as a panic button.
To compare it to the only other truly Epic monster I've assigned a LA to so far, the solar (no, a colossal animated object does not count). The solar has an ECL of 24, but is obviously stronger than its demonic counterpart, if only because of its cleric casting.
In fact, despite its many abilities the balor does lack one thing: customizability. A player won't be able to add much except in the most Epic of games, and even then it's questionable how much extra levels will increase your strength comparable to a wizard, or a beguiler, or even an optimized barbarian.
+0 LA, partially because of feedback I received.