I will pick Giratina. But only its Origin forme. Don't know why but its Altered forme seems weird to me and I love seeing Giratina around the Distortion World.
Being Giratina my first one, the second would be Palkia. I have to say I have only played Pokémon Diamond and Platinum, but Palkia has something that I like. And I think I got tired of seeing Dialga everytime I played Diamond so that could be another reason why I like Palkia more than Dialga. And isn't it cool the combination of Water/Dragon type?