If I could choose anything, it would likely be Celebi for the reasons that have already been mentioned. Powerful psychic attacks combined with an affinity for nature, along with the powers of flight... and time travel of course. How could anyone forget about that? Celebi's a powerful but light hearted Pokemon, and that's just dandy.
Of course, not everyone can be a legendary Pokemon. I kind of like the idea of being a Sigilyph. A powerful, ancient Pokemon who has seen the Unova region wither and grow time and time again. Plus it has a pretty decent movepool. ;)