The skindancer: a horrible monster whose ever-shifting defenses provide protection against everything, with the notable exception of an airborne Wand Of Magic Missile-owner.
The bad first: fifteen monstrous humanoid RHD, and only 22 strength and Large size to show for it. Great constitution, terrible intelligence and charisma, reasonable dexterity and wisdom. +7 natural armor, as well as a natural ability that grants another +6 deflection. Finally, two 2d6 slam attacks which nobody cares about because these things can use weapons.
The skindancer's special abilities are unique, if nothing else (notable exception: Trip, which requires using its slams and is very bland and weak). Adaptive Energy Resistance gives it resistance 20 to whatever was the last energy attack to damage it (per the rules compendium, this covers acid/cold/electric/fire/sonic), and Damage Reduction gives it DR 15 to the damage types of the last weapon attack that hit it (so DR 15/slashing if the attack was with a morningstar, DR 15/bludgeoning and piercing if it was with a greatsword, and so on).
I'm not sure how strong those abilities are, but they do seem to have some utility. That said, the few CR 15 foes I sampled did usually have ways to get past those resistances, and there's still the issue of a defensive ability just not being very strong without offensive pressure or a means of drawing attacks.
I think that in the end I'll go with -0 for the skindancer, but it's a neat concept that could definitely be viable if it had a few less HD. Do discuss.