When it comes to as powerful as Kaiba. He's an expert player, and is one of the few that has the power to stand up to Yugi Muto.
Kaiba is on the level that he is for quite a few reasons, but one of the most major is his use of powerful spell cards. He knows how to make a strong deck, and he even used quite a spell cards multiple times throughout the show. As a note, all the below effects are their anime versions.
Enemy Controller
Enemy Controller is truly one of the most dramatic cards in the entire series, requiring the to input one of two different commands to either destroy or take control of an opponent's monster for the turn.
This is very useful in order to summon a monster that needs multiple tributes. Maybe something like a Blue-Eyes White Dragon, perhaps?
Ring Of Defense
Ring of Defense is a slightly specific trap card that lowers all effect damage taken from a trap card to zero. This card can certainly be useful in certain situations, but there are plenty of other cards that are more helpful and can be used in broader situations.
Nevertheless, this is a card that Kaiba has opted to keep in his deck on multiple occasions for increased protection.
Soul Release
Soul Release is a magic card that allows the player to remove five cards from their opponent's graveyard from play, which can be a really strong strategy.
Kaiba is all about removing cards from play and bringing them back onto the field, meaning that Soul Release is a strong card for him to use, especially if he can figure out a way to bring those cards back onto the field on his side. Though that isn't always his strategy with this card.
Negate Attack
Negate Attack is a strange card to make it onto this list, but in the original anime, this card is actually treated as a magic card for some reason, despite it being a trap card in the real game and in the dub.
Nevertheless, Negate Attack is a strong card that allows the to immediately end their opponent's battle phase before any attacks are completed, which can certainly save the player in a pinch.
Cost Down
Cost Down is a magic card that allows the to discard one card from their hand to decrease the star count of every monster in their hand by two for the remainder of the turn.
That means that a 5- or 6-star monster won't need a tribute in order to be summoned, and a 7- or 8-star monster will only need one tribute in order to be summoned. No matter its intended use, Cost Down is a super useful card.
Pot Of Greed
No memes are being referenced here, let's get that out of the way right now. No. Anyway...
Pot of Greed is a magic card that allows the to draw two cards from their deck, which is obviously an insanely powerful effect, which is why the card is still on the real-life ban list to this day. Despite this, basically, every single duelist on the show uses this card, and it's a mainstay in everyone's deck.
Flute Of Summoning Dragon
Flute of Summoning Dragon is the perfect card for dragon-master Kaiba. It is an incredibly powerful spell card that can only be used when Lord of D. is on the field. When that monster is on the field and Flute of Summoning Dragon is used, both players can summon up to two (Blue-Eyes White) dragons from their hands.
This is obviously useful for Kaiba. While his opponent may or may not have dragons in their hand, Kaiba uses those monsters almost exclusively.
Card Of Demise
Card of Demise is a hugely powerful card that allows the player to draw up to five cards from their deck. After 5 standby phases have ed, they must send their entire hand directly to the graveyard.
This card is so, so powerful, but the drawback is also pretty large, too. However, the way Kaiba uses it, he usually finishes the duel before the cost would ever need to be paid.
Polymerization
Polymerization is a card that has been around since the beginning, and it has always been in Kaiba's deck. It allows the player to fusion summon a monster provided that they have all of the materials necessary in their hand or on the field.
Of course, this card is most often used by Kaiba in order to summon Blue-Eyes Ultimate Dragon, his ace card. How would Kaiba ever be able to leave a card like this out of his deck? It's a stand-by for him, but it isn't No. 1.
Monster Reborn
Just like Pot of Greed, Monster Reborn is a card that just about every duelist has in their deck, and why not? It is an incredibly powerful card that allows the player to revive a monster from either player's graveyard and use it on their side of the field.
It's a powerful card that makes sense in any deck, which is why Kaiba uses it in almost every single version of his.
