I would most likely have Uxie. It is said to be the most intellectual of all Pokémon, so every time I got myself into any sorts of trouble, it would always be able to figure out a way to help me get out of it. Uxie also has access to a variety of moves that'd be able to aid me throughout my life, such as Protect, Heal Bell, and Future Sight. Future Sight would enable this Pokémon to detect what lies ahead of me in the future, so that I can always be prepared for just about anything. With Heal Bell, this Pokémon could keep me healthy at all times—mentally and physically. Protect would allow Uxie to keep me away from being hurt, and stop me from getting into any kind of a major accident. These moves may not be very useful when it comes to battling (well, unless you use them strategically, of course), but they can certainly be of great assistance in daily life. d:
I'd also like to have a Vulpix, if possible! It may not be as helpful as Uxie, but every time I got to see it, it would always be able to brighten up my mood. :) Vulpix are extremely cute and cuddly, and they're very easy to befriend!