I would definitely be a phoenix – while in phoenix form, I would never die; I would have healing powers and I would be able to sing wonderfully. Besides for being able to fly with large weights. (Supposing it is possible to give oneself powers of another creature when turning into an animagus – although one could suggest that transfiguration by a human will only externally change the shape and size but not the actual ‘human’ creation. I am not sure)
BTW – why, once one is an animagus, can one not transform into more than one different creature? Sirius was always the same dog, Pettigrew was always the same rat – why does this necessarily need to be so? If one is just performing complex transfiguration, why can one not customize it each time?
And if Moody can turn Malfoy into a ferret, why can he not transform himself? (Although maybe Animagus, which is **somehow** performed without a wand (?) is harder.)