I would be a simple house Cat just so I can know the luxury of being a Cat and having no responsibility and being fed when i'm hungry and someone to clean my litter box. Plus I would do the most random and ridiculous things, like watch t.v or something stupid.
I wanna be a megalodon, cuz they are very strong sharks, even though they are extinct, if you dont know what megalodon is, its right here: http://en.wikipedia.org/wiki/Megalodon