From Yu-Gi-Oh!

Underneath his jacket, Bastion wears a special vest that holds six different Decks, one for each Attribute. He changes decks accordingly depending on his opponent, although all of his shown Decks contain cards based around scientific processes.

Anime

Misc Decks

It's unknown which of his six decks each of these cards belong to, and most of them were seen when his cards were found floating in the water.

Water Deck

His Water deck is a Water Dragon Deck which focus on tributing Hydrogeddon and Oxygeddon using Bonding - H2O to special summon Water Dragon.

7th Deck

This deck is based on his Water Deck and is built specifically to counter Jaden Yuki's Elemental Hero Deck. It is explicitly stated to be a separate deck in the original anime, which he calls his "7th Deck." No such distinction is made in the dub.

Earth Deck

His Earth Deck's theme is the manipulation of plus and minus electronegativity.

Water/Earth Deck

In Seasons 2 and 3, it's stated that Bastion combined elements from all of his decks into a single unified concept. However, only elements from his other shown decks, Earth and Water are shown.

Manga

Yōkai Deck

In the manga, Bastion plays a Yōkai Deck, which relies on the Summoning of monsters from Japanese folklore from his Graveyard. Curiously, the theme of these decks totally differs with the anime decks, which are Science-related, while manga decks are based on Japanese mythology.

Fire Deck