Digimon series: Wargreymon.
I would immediately kill anyone who I didn't like and maybe blow up California and New York; wait, I WOULD blow up California and New York (maybe Chicago while I'm at it). Hey, Digimon can go between the Digital World and the real world, so I could do whatever I like.
Maybe I would blow up some military bases first. Then I would de-evolve to Agumon when they start flying after me and hide in a tree. It would instantly go on the news that there was some gigantic dragon-man creature that defied gravity, science, and the laws of life. I would evolve back up whenever I felt like having some fun. What if they eventually got me and killed me?
I'd be an egg again and would eventually grow up to be Wargreymon again.
Pretty much near-immortal being with awesome power... Sounds like fun!
'Course if there wasn't the whole immortal chain or being able to create tiny meteors out of nowhere, I would probably be...
...Snake. 'Cuz he's Snake.