Yo dawg, I herd you like mouths, so I put mouths in your mouths so you can scream while you scream.
Gibbering Mouthers: aberrations with four HD, too many teeth, and an apparently TN alignment (because killing sapient beings when you don't have to is totally not evil).
Their base chassis could be better, but it's not horrible either. Ability scores are mostly either small bonuses or huge penalties (-6 intelligence), but constitution has a whopping +12 bonus. Mouthers also have a huge natural armor bonus (equivalent to full plate), a whoppin' six bite attacks (admittantly weak ones, but mouthpick is a thing) and even some DR.
Their special attacks are interesting too. Spittle is apparantly useable as part of a full attack in addition to being usable for free once per round and can inflict blindness for several turns, Gibbering is a big save-or-suck that your allies are protected against with some trouble, and Ground Manipulation basically robs everyone nearby of their move action. Oh, and being amorphous immunizes you to a few common attack forms.
The Improved Grab + Swallow Whole + Blood Drain trifecta seems pretty bad at first glance, as the mouther is a medium creature with no inherent strength bonus. However, advancing it by a single HD makes it become Large, adding a whopping +8 to its grapple modifier.
All considered, I think +2 LA is best here. In addition to the advancement shenanigans already mentioned, mouthers are simply a strong monster with lots of useful abilities. I admit I've strongly considered a +1 LA, though.