The planetar, infamous for getting 17th-level cleric casting and a bunch of other goodies at CR 16. Apparently the low HD was supposed to balance it out?
This casting makes rating them quite difficult, though, and my eventual decision surprised even myself. In hopes of making it understandable, I'll evaluate possible LA's one by one.
+0, +1, and +2 LA all have the same problem; they allow epic spellcasting by ECL 20, so they're out. +3 LA removes all differences between a planetar and an equally-leveled cleric's casting, but the cleric doesn't have the additional perks of being an angel. A monster being stronger than a tier 1 class is a pretty clear indication its LA is too low.
+4 LA, then? It'd place a planetar one level behind a cleric when it comes to casting. However, once you get 9th-level spells, all more caster levels do is give you additional slots, while the planetar still has its regeneration, spell-likes, SR, etc.
Remaining level adjustments: +5 and +6. I actually think both could fit, and the only reason I pick +5 is my dislike of characters without any class levels.