Localizing names in anime is tricky, and perhaps no anime is known for localizing names for western audiences more than Yu-Gi-Oh! Some minor characters having their punny names changed, like "Dinosaur Ryuzaki" to "Rex Raptor," make all the sense in the world. His name "Ryuzaki" has the word "Ryu," meaning "dragon" in Japanese. Many exclusively English-speaking fans wouldn't get the pun in his name, so the franchise fully committed to the dragon theme with the dubbed name.
However, some name changes are either pointless or just needlessly westernize a normal Japanese name. Maybe changing "Jonouchi" to "Joey" would have made sense for the franchise in the early 2000s, but kids and teens are growing up with eastern media more and more, and changing the names only serves to create a barrier between subbed fans and dubbed fans. The worst name changes are pointless at best and actively harm how the character is perceived at worst.
Gong Strong
Gongenzaka Noboru was the Yu-Gi-Oh! "Joey Clone" of Arc-V. He was a headstrong best friend to the protagonist who practiced his own style of dueling without using any spells and traps.
Yu-Gi-Oh! is known for having characters with punny names, but "Gong Strong" isn't really a fun pun, it just makes an important player in the series seem like more of a joke character. Names like this are usually reserved for minor antagonists, not major characters that are present for the entirety of the story.
Declan Akaba
Reiji Akaba is the primary rival of Arc-V and the character who gathers all the protagonists to take on the evil Fusion Dimension.
Part of what makes the name change so unfortunate is the "language barrier" it creates between sub and dub fans. Having a joke character's name change is one thing, but Reiji is arguably the second most important character in the entire story. Also, it's hard not to agree that Reiji sounds more menacing than Declan. What further makes this name change baffling is the surname "Akaba" being kept while "Reiji," a name that sounds so similar to the western name "Reggie," was changed so drastically.
Bronk Stone
Tetsuo Takeda was many fans' least favorite part of Zexal, a great Yu-Gi-Oh! series that people often dismiss because of a poor first season. Tetsuo was a friend of Yuma who eventually met his end at the hands of the Barians in the final season.
Bronk's name change was both unnecessary and gave him a last name that was cooler than he deserved. Nobody named "Stone" should have an ace monster like the goofy "Tin Archduke."
Akiza Izinski
Aki Izayoi was one of the three main characters of 5Ds, a powerful signer, and Yusei's implied love interest. She's also the user of the iconic Yu-Gi-Oh! monster '"Black Rose Dragon"
The strangest thing about this name change was how little it actually changed. There was no real point in adding an extra syllable to Aki's name, and "Izinski" isn't that much easier to pronounce than "Izayoi." Of all the name changes this one feels the most pointless, but at least Sub and Dub fans will know what the other is talking about thanks to the names being practically identical.
Zane Truesdale
"Kaiser Ryo" was the strongest duelist in all of Duel Academy, not even Judai could match him as a duelist in season 1. The "Cyber Dragon" user was the driving force of many of GX's best duels.
The saddest thing about the Dub's name change is more the loss of the nickname "Kaiser" than the actual name change. Throughout the series, it was a title that Ryo wore proudly, and there was never really a replacement title like "Emperor" that the localization latched on to. Just as many pro basketball players go by nicknames like "Magic" or "King James," it always felt like a neat addition that a pro duelist like Ryo had his own similar flashy nickname. The Dub took that aspect of his character-building away by just calling him Zane.
Mai Valentine
Mai Kujaku was a major character in the Duelist Kingdom, Battle City, and Waking the Dragons arcs of the original series. After a traumatizing experience against former Yu-Gi-Oh! antagonist Marik, Mai accepted the power of the "Seal of the Orichalcos"
The strangest thing about Mai's name change was that it broke the naming convention of the first season: the supporting cast all had names that fit their decks. Most western audiences wouldn't know that Kujaku means peacock, so a name change was helpful, but the Harpies weren't especially a love-focused archetype so the choice of "Mai Valentine" was strange. It really only seemed to cement the character as merely being Joey's love interest.
Jaden Yuki
Judai Yuki was the protagonist of GX and the character that first introduced the beloved "HERO" archetype to the game.
The name change itself is fine, Jaden Yuki is a fine name, but it's a general rule that protagonists' names should never be changed from sub to dub. It's a rule that the franchise has kept up since 5Ds, and Judai remains the only protagonist who has had their name altered. The saving grace is that the iconic "Yu-" in the last name wasn't changed.
Zuzu Boyle
Originally known as Yuzu Hiiragi, Zuzu Boyle uses the Melodias archetype and is the main female character in Arc-V. Yuzu is later shown to be one of four duelists who share part of a legendary duelist's soul.
Taking the "Yu" out of a character's name in Yu-Gi-Oh! is like stripping a title from them. It always felt like part of the reason Yuzu was giving the same "Yu-" prefix as every protagonist was to emphasize her importance in the story, and changing that damages the way the audience perceives the character.
Vellian Crowler
Chronos de Medici was Judai's professor and a secondary antagonist of the very first arc. Professor Chronos didn't get a lot of duel time, but he was a key character that was on screen for much of the series.
Since supporting character names that reference the deck they play have always been a huge part of Yu-Gi-Oh!, Chronos had the perfect name for a character that played a deck aesthetically centered around old clockwork and gears. Crowler was a neat name, but "Professor Chronos" was one of the best names a supporting character has ever been given.
Ray Shadows
Vector, also known as Rei Shingetsu in the sub, is the secondary antagonist of Zexal. In order to invade the human world, Vector disguised himself as the space policeman Rei Shingetsu and did his best to gain Yuma's trust.
If a superintelligent space alien was trying to gain the trust of a middle schooler, the last name "Shadows" was a questionable choice. Not only does the name make Vector seem like a much cornier villain than he was in the sub, but Yuma seems like a complete fool for trusting him in the first place.