If I had a choice? Probably Dragonair, actually.
Dragonair are beautiful and awesome, and I just really like the idea of being a dragon-type since dragons were totally my thing when I was little. I grew up playing the old Spyro games and when I first played Pokemon, half my team was Dragon I spent a ton of time drawing dragons, too. So that's definitely a concept that appeals to me.
Another good thing about it is that I would be rare enough so that nobody could just come along and catch me whenever they feel like it, but also that if I did decide to pursue a lifetime of battling I could evolve into the badass Dragonite and wreck everybody. :3