What I "would" be is Cubone, according to some internet test. That was a while ago, though, and I've changed since then. So I doubt it would be the same still.
I probably would want to be something like a Rattata. Seems fun to me.
In the long run, though, something more like Liepard would probably be better. Still very agile, so I would be able to get around quickly, but not as weak as Rattata.
If the idea is just to have fun and rampage, though, then Charizard. [:
I would want to be a Sceptile because of the awesome move that is Leaf Blade. Never again would I ever have to struggle opening my cans of corned beef hash for breakfast.
Objectively though, I'd say it boils down to a Growlithe, a Krabby, or a Sudowoodo.
I'm a bit of a dog in real life, and Krabby and Sudowoodo are zany, so I guess those fit me more to a degree than others.