The
Steel Dragon.
It's a social butterfly, using it's abilities to mingle with other species instead of hanging out by it's self in some cave. Which also means the Encounters for it will be a little unique, taking place in urban areas where the local law enforcement will probably see the adventuring party as the violent party. It's flavor entry is also hilarious. I'm sure we've all heard about the merchant that turns into a dragon and eats someone for trying to haggle prices the wrong way. Well, the Steel Dragon does that and gives you that excuse for wanting to
From a PC stand point, it's also the most powerful TD for optimization. Having the lowest HD to casting ratio and eligibility for total LA Buyoff. Other highlights include two Breath weapons, one Acid typed which is awesome and the other poison that deals Constitution damage giving you an alternative offense, and it also has full access to the Cleric Spell List with the Trickery & Knowledge Domains which is pretty nice. And it has 5/day Alternate Form (FR:DoF>online) with an unlimited duration and you're allowed to choose any Medium or smaller Animal or Humanoid shape. Since Human's bonus Feat is a Racial Trait lost if they Polymorph out, then it's fair to gain one if you Polymorph in so that's a 5/day swapable Feat slot for lots of versatility.
