BLUE EYES WHITE DRAGON
Kaiba's cards are... infinite! XD
He's one of the richest man in the world, so he can buy what he wants and in large quantity. In many occasions Seto shows his cards' handbag, rich of the strongest monster, spell and trap cards that Pegasus created.
But his game is another thing. Kaiba based it on a specific strategy, that changes duel by duel. But even if his strategies are different, he always bases his game on a special monster, his favorite, that he considers mirror of himself and his obsession: Blue Eyes White Dragon.
TRAP & SPELL CARDS AS STRATEGY
This is the card of Seto's life, that marked him in soul since his past life, when he was the High Priest Seto, the one who offered as sacrifice Kisara, the girl that hid herself in the White Dragon's spirit, in order to obtain its strength.
5000 years later, Kaiba obtained this card with bad methods, but he didn't know that behind a simple card, there are his past and his destiny. Blue Eyes White Dragon is the top of his strategies, each one of them is based in order to summon it. For this reason, Seto's game is based in many Trap Cards, that allows him to overturn the situation using the opponent's monsters and cards. Also, Magic Cards are very important, with their help he can play his White Dragon without sacrificing two monsters, and combine it with other two of his dragons or other cards.
For a period he was obsessed with God Cards and used Obelisk the Tormentor as biggest monster, but playing, he understood that the only card that makes him the winner, can be Blue Eyes White Dragon. Against Isis he prefers to sacrifice Obelisk for playing the dragon, and this action is Seto's winning choice, that destroyed Isis' visions and allowed him to win the duel. Even if Seto never admits that Blue Eyes White Dragon is his own destiny, he's forced to accept the connection with his past, that now more than yesterday, is changin' his life.