The infamous allip: best known for being able to
killpermanently incapitate half the monster manual rather easily, with most of the other half being unable to hurt it. I don't like saying it, but perhaps WotC was right when they didn't give this a level adjustment.
So what does being an allip give you? Four undead HD, the undead type, incorporeality, turn resistance, darkvision, fly speed at perfect maneuverability, the ability to make people who try to read their minds go mad and a massive AoE that makes people temporarily go insane. Oh, almost forgot: they can also drain your sanity with a touch. Also, an allip's ability scores bonuses are pretty good, and their skill list contains all the scouting skills.
So where put the allip's LA? A random sample from the monster manual reveals that monsters start being able to reliably beat an allip from CR 8 on, implying a LA of +4. However, comparing allips to PC's, I personally think +3 LA is more suitable, especially considering my initial search failed to account for magic weapons: something easily available even for a standard EL 7 encounter.
Note that this decision was not easy, and I would love some feedback here. Allips are tricky enough to use as DM's: making them viable PC's will be even more difficult.