At the very bottom of the abyssal food chain are the dretches. I don't understand why you'd want to play a being whose main purposes are serving as snacks for greater demons or getting summoned by novice demonologists, but who am I to judge?
Dretches have two outsider RHD, pretty good natural armor and decent natural weapons. Their ability scores are average, with the strength and constitution bonuses offset by the huge intelligence penalty. The problem here is that if you're focusing on strength, chances are you won't be small-sized, and if you're a small-sized melee combatant you'll be relying on sneak attack or spells for damage, both of which the dretch will be behind on.
Except for all that, dretches possess a weird form of telepathy that only works on creatures able to speak abyssal (though it should work with mindsight) damage reduction, energy resistances and two solid spell-like abilities.
You can also summon another dretch once per day, but it only works about one in three times. Fortunately it sticks around for an hour, making it a viable ability to use (or fail to use) before an important battle.
Finally, on dretches and weapons: it's not clear whether they can use them. If they can, it considerably increases their versatility, but if they can't you'll have to rely on a mouthpick weapon instead. No source I consulted reveals anything about this matter.
Taking all that into account, what LA should be given? +0 seems reasonable because a dretch doesn't really give a lot. Natural armor and weapons can be gotten more cheaply, and by 5th-level all your tricks are duplicable by a half-decent caster. A lucky casting of stinking cloud may end the encounter, but what will you do the rest of the day? Deal laughable damage with natural weapons? Get sneak attack from some source, only to fall behind in damage to an equally-ECL'ed rogue?
Of course, there may be people willing to give up two levels for the impressive array of protective abilities a dretch grants, but they shouldn't be further punished for it. +0 LA.