The quasit: another of the odd demons that lacks the tanar'ri subtype.
Quasits have a reasonable chassis for a creature of their HD. They receive bonuses to dexterity and wisdom and a penalty to strength, have weak natural weapons (with semi-useful poison) and some natural armor. Their fly speed is great, their landspeed adequate. Finally, there's fast healing, DR, poison immunity and fire resistance: all good things to have.
The tiny demons possess a few SLA's: Detect Good probably won't be so great (even in an evil campaign, many foes will be nongood) but Detect Magic is useful to have (as long as you don't have a warlock in the party). Self-only invisibility is wonderful at this level, especially if you can find a way to damage foes without breaking it (I find reserve feats useful there).
They can also use Cause Fear with a 30 foot-radius once per day, which you'll get to use a few times before it gets obsolete. Sadly, due to the way PC and monster HD fail to match up and the awkward radius, you'll send your allies running more often than your enemies.
Their last SLA, Commune, can be very impressive or very underwhelming, depending on who's behind the screen. The utility of the spell is highly DM-dependent, after all. It can either be a limitless source of information, or it can be the DM pointing you towards the plot you would've stumbled upon anyway.
Finally, there's Alternate Form. Quasits get to pick two forms from a list of five. I would pick wolf and centipede myself, but it depends on your campaign. The alternate forms are unlikely to give you something you don't already have, though.
The various abilities a quasit gives, combined with its excellent RHD, make +0 LA inappropriate. I'll put them at +1 instead.