For every Lawful Good hero in the anime/manga, there's a chaotic evil around the corner ready to plunge the world into darkness at a moment's notice. So let's dig into the moral alignments of the main heroes & villains to see where they land.
10 Yami Yugi/Atem: Lawful Good
Being a ruler of Ancient Egypt, it's pretty safe to assume that Yami Yugi/Atem fits the bill on the lawful side. He lives by a strict code of honor and expects the same out of other duelists and friends he encounters.
Even when he has lost his memories at the start of the series, he is compelled into action and to save the innocent soul of Yugi's grandfather and later on, the Kaiba brothers. Despite Dartz's attempts at clouding his mind, Atem was indeed a just and noble king which secures him as a lawful good character.
9 Yugi Muto: Lawful Good
While their temperaments and attitudes way be worlds apart, Yugi Muto is just as a lawful good as his brother from another time Atem. Just like the Pharoah, Yugi always sees the best in people and will give the shirt off his back to help someone out if he thought it was the right thing to do.
As far as strict codes go, while it may be funny to think about from a meta perspective, Yugi does hold a true belief in the "Heart Of The Cards" which has served him extremely well in his dueling career. He may be shyer than his spirit brother, but Yugi is a lawful good soul.
8 Seto Kaiba: Lawful Neutral
When it comes to Seto Kaiba, it truly depends on what point of the series we're talking about. If it's before the mind crush, he's clearly a chaotic evil as he set up an entire amusement park designed to kill Yugi and his friends in the manga. However once his troubled soul was shattered to be reassembled, he becomes a great lawful neutral character.
Although he can be extremely harsh at times, Kaiba does indeed live by a code which he adheres to in order to become the successful CEO and duelist he is. Kaiba is many things, but he is no liar which fits lawful neutral characters. You can't quite call him a "good" character though as he is willing do some less than noble acts if he has to.
7 Maximillion Pegasus: Neutral Evil
For the purposes of this list, let's discuss Pegasus during his time in Duelist Kingdom as that's where he was most important to the series.
By far the most entertaining of all the villains in the series, Pegasus is laid back and carefree compared to evils like Bakura and Marik. Despite being dead set on the goal of bringing his wife back to life, he carries himself with a devil may care attitude that allows him to swing between his rulings as the CEO of his company (acting more lawful) or as wielder of ancient evil powers (chaotic).
One could argue that he has a noble goal, but his methods and absolute carelessness for whoever he crushes to get his way makes him a surefire bet for Neutral Evil.
6 Yami Bakura: Chaotic Evil
A thief. A would-be destroyer of worlds. A villager hell-bent on revenge. What else could Yami Bakura be but chaotic evil?
Of all the characters on this list, Bakura is the easiest one to place on this alignment chart as his ultimate goal is nothing more than plunging the world into darkness and killing off humanity. Granted, being tied to greatest evil the world has known (Zorc) helps with that, but nevertheless. No move is underhanded or evil enough for Bakura to consider doing and he will manipulate or dominate anyone he can to achieve his goals.
5 Mai Valentine: True Neutral
When compared to the other bombastic and over the top personalities on the show, Mai Valentine is probably the most reserved of the lot. Mai keeps a cool head during most of her duels and in her daily life and that's probably because of her status as a true neutral.
While she sways slightly to the side of good as any true neutral would, this doesn't stop her from acting in her own self-interest most of the times. She'll commit good deeds on a moment's notice, but she doesn't adhere herself to some grandstanding moral code like Yugi or Lawful Good/neutrals might. When she even felt down and out, she turned to Dartz's organization for help.
4 Tea Gardner: Neutral Good
The biggest cheerleader in Yugi's group of friends, Tea Gardner is someone you always want to have in your corner. While an argument could be made that she is a lawful good like her crush Atem but she is much more willing to break ranks and rules if it means that she can help out the innocent and her friends.
When something is wrong, Tea is likely the first person to speak out about it even when being more reserved would be a better option. Tea will do good for goodness' sake, a great quality and a trademark of Neutral Goods.
3 Joey Wheeler: Chaotic Good
While other duelists like Kaiba and Yugi have dueling strategies in place and live by lawful alignments, Joey Wheeler is anything but lawful. The scrappy underdog of the series, Joey quickly turns from the bully of Yugi to his best friend. Unlike others who may have a care or who might think twice about sticking their nose in something, Joey is always to first one to break rules if he thinks it's the right thing to do and will do so without a second thought.
Technically Joey shouldn't have jumped out to protect Mai during her duel with Marik as it was against Battle City rules, but Joey didn't give a damn if it disqualified him. Joey's troubled childhood with an alcoholic father and frequent interactions with thugs on the streets have shown him what it takes to survive and that is a streak of chaotic goodness.
2 Marik Ishtar: Chaotic Evil
A spirit born entirely of rage and bitterness, Marik Ishtar was a boy who strived to be free from his lifetime of servitude to a dead king. He wanted to ride motorcycles and enjoy the life that every kid should have. However, because of his family's duty to Pharoah, he was never allowed those freedoms until he killed his father and unleashed a dark alter ego which would threaten the world.
Despite leading the Rare Hunters, Marik was quick to dish out severe punishments to those who failed, seemingly taking pleasure in unleashing his rage upon his underlings. When that alter ego comes forth (which remember was birthed from his own emotions), it takes delight in shadow games and torture. Chaotic evil, easy.
1 Dartz: Lawful Evil
Lawful evil characters in Dungeons & Dragons often hold allegiance to an evil deity who they pledge servitude to so that they may carry out their will. Remind you of a certain filler arc villain?
Not only does Dartz's corrupted soul do everything in his power (including sacrificing himself) to bring the return of the Great Leviathan, but he was also once the ruler of Atlantis which further adds to his lawful nature. His plans of luring Raphael, Alister and Valon to his side shows a devious streak common to villains of this alignment.