Angel, Astral Deva
The astral deva is the first monster I evaluate that already has a LA. Said LA is so high that it brings back the problem of giant health gaps between party members, which is usually only an issue at low levels. Said LA is so high, that by the time you can finally play an astral deva, the party's wizard has been binding them for nine levels and shapechanging into them for three. Said LA is so high, a commoner of equivalent level could duplicate most of their abilities with WBL only.
Level adjustment notwithstanding, astral devas are a strong race. +8 or higher bonuses to all ability scores, high speed, high natural armor, damage reduction, energy immunities, spell resistance (high enough to remain relevant even at 20th-level), permanent Magic Circle against Evil and Tongues, shapechanging, uncanny dodge, and more spell-like abilities than you shake a +3 heavy mace of disruption at.
So what level should it become playable? Devas are offensively behind dedicated melee builds, though they try to make up for it with a fair deal of casting. The defensive abilities won't be heavily factored in, because being good at not dying isn't great if you fail to establish yourself as a target (just ask monks) and again, spell resistance will make you resist beneficial spells while offensive spells can just be directed at different targets.
In the end, it is obvious that despite their big numbers, devas simply fail to pick a niche and fill it well. They definitely aren't useless, having a wide range of abilities, but they lack the strength a focused PC build possesses when doing its chosen job. I put their LA at +2.