My quick answer would be dolphin, just to have that portable therapy session for whenever I'm not feeling very pretty and happy.
After deep consideration and meditation, though, I would have to choose the peacawk.
*Laughter*
*Applause*
Thank you, but I'm not kidding. Just visualize this scenario; Joe Blow the accountant is coming home from work in his car. It's been a long, harrowing day. Verbally abusive co-workers, terrible coffee from which he's finally feeling the caffeine crash from, and a morning headache that's been steadily throbbing harder each hour. However, work day's over at least, and now he could go back home to... what? A wife who he suspects is cheating on him with his firefighter brother? A surly teenage son who makes no attempt to hide his packs of cigarettes? An infant daughter who only ever giggles whenever she sees her father tripping and stumbling on his own feet? And then to cap it all off, his car breaks down in the middle of the road. Much too exhausted and impatient to call a tow-truck and wait, he decides to walk the rest of the way home. And that's when he sees it, smack in the middle of the street.
A beautiful, plump and magnificent peacawk. In the suburbs! But there's no zoos anywhere near this city, so how...? The peacawk will gaze at him solemnly, silently contemplating him, and will croon softly, "Find joy in the little things, ma boi." And it will fly off, leaving Joe flabbergasted. He'll continue walking the way home, but inexplicably there will be a spring to his step as he wildly entertains the idea of skipping daintily the rest of the way home. He laughs it off and doesn't decide to skip after all, but the important thing is that he even considered it.
This is my legacy. Bringing hope to people who have forgotten the taste of its sweet nectar. Helping the helpless. The rest of you can enjoy committing acts of perversion and theft in the disguises that the morphing ability will enable, but personally, this is what will bring me satisfaction and inner peace.