This has always been something I've had trouble answering. There are lots of Pokémon I'd like to have, I always narrow it down to about 3 though.
Gardevoir, Start with a Ralts obviously. Gardevoir would be like a really close friend who's with me all the time. I'd talk to her about loads of stuff, she'd always know how I was feeling 'cause of her emotion based powers. Also I'd like something pretty x3
Jirachi, because it's small and cute and would be fun to play with and carry. It would be a bit like a pet I could connect to on an emotional level. Also maybe it'd let me wish for stuff :P
Flygon, because I've always wanted to fly, and I've always wanted a Dragon. Really, Flygon is my favourite dragon in the series by quite a longshot. He's also kinda a huge bug, which is epic.
I try to narrow it down after that but it becomes impossibly hard to choose between Flygon and Gardevoir ._.