There are a couple things i'm tempted to play:
Tsochar: Scout, with a limited polymorph gained at level two, and who later gets casting. Nice hit die, skill progressions, and natural weapons. Has fine manipulation. The stat bonuses don't match up with what they traditionally do (Multiclass into Sorc when they get strong enough. He recives an Int bonus rather than a Cha one), and unless I play a Tsochar Totemist, I won't be able to do the headcrab routine very often. Might work on a rogue-like class because of his excellent infiltration abilities. Has a climb speed, and combined with Hide and Move silently, can perform some hideous terrible things. Think Ezio meets the Xenomorph. It can also wear Three amulets, four bracers, and four rings, basically being made of arms and necks (It says this in LoM anyway. I would prefer regular body slots)
Mindflayer: Not that good as a caster, strangely. I was thinking of dipping for the tentacles and using it as part of one of the Dicebag Psychic Charger builds that normally take Illithid grapple 4 times. If I level up far enough, I get a bunch of SLA's, but they really aren't useful. Illithid Warblade could be fun though.
Shoggoth: Swimming, water breathing, climbing, full BAB, heavy hitting natural attacks, fine manipulation. As I level up, I can cause insanity in nearby creatures (So obviously you all need Immune:Mind affecting if I go this route). Inflicts grapple that cannot be escaped, and then eats them.
Werewolf: I bite, things die. Also free werewolf leadership! We have a melee though.
Sarrukh:
Ak-Chazar Rakshasa: For when leadership is not enough, you call the army of darkness. Turning, and 8/10 Sorc casting. Makes an ideal Necromancer. Basically dread necro, the monster. I tend to shatter games with this.
So! Take your votes ladies and gentlemen!