Favourite legendary would have to be Suicune or Groudon. Suicune is just calm, collected, pure and just got an aura of beauty about it. It's majestic, but it's also really strong, and I like that. Groudon is the complete opposite - a powerhouse, not calm, collected or pure whatsoever, and the only aura about it is destruction (though we only see that because of Kyogre in fairness.)
As for non-legendary, I can't really decide. Everything in my trainer card apart from Salamence pretty much ties for top spot. No real reason, I just think they're all cool in their own way.