Celesteela, by far. I think they're all cool, but Celesteela is based on one of my favorite Japanese folk legends, that of the
lunar princess Kaguya and the bamboo cutter (hence the bamboo design). She's also based on a Japanese lunar orbiter SELENE (aptly nicknamed Kaguya) hence the rocket arms. And who can forget, her Japanese name sounds absolutely badass- tekkaguya (tekka(gunfire)/tekko(steel)+ Kaguya (moon princess)).
She's also got the strongest Heavy Slam in the game, due to her being tied with Cosmoem for the title of heaviest Pokemon at a whopping 2204.4 lbs/999.9kg. Even after using Autotomize, she still outweighs most Pokemon, so the power of Heavy Slam is still ginormous with +2 towards Speed.
That being said, I'm a little upset that her GX card is as disappointing as it is. Her regular attack (1 steel 2 colorless for 30+30 for each energy in the retreat cost) will be doing all of your damage, although probably not much. It hits Gardevoir's weakness, but that only reaches 180 without a CB. It hits Golisopod-GX for 120 and Zoroark-GX for 90, which is okay, but not worth risking 2 prizes over IMO. And the GX attack does hit hard, but Town Map effect could be pretty much anything else and I'd be happy.
Finally... I think she has a cute smile (`・//ω//・´)
(Edit: replaced an instance of 'disappointed' with 'upset' because the sentence seemed redundant)