The Pokémon anime's Masters Eight determines the strongest Pokémon Trainer in the world. As of the final arc of the Journeys series, the current roster definitely lives up to that description. In addition to Alain, Ash's only main rival to have won a Pokémon League, it consists almost entirely of regional Champions.
That being said, while the roster can only list eight Trainers at a time, the anime has shown many other characters that can throw down with this formidable group. The Masters Eight is constantly shifting, and plenty of those who missed the cut have their own reasons for doing so. In the end, the final eight are far from the only ones worthy.
10 Flint Is The Anime's Strongest Elite Four Member
Of all the Elite Four members shown in the anime, the one given the most focus was Flint, the Fire-type expert from Sinnoh. Flint demolished Ash's team in a complete sweep during Diamond and Pearl, with only Pikachu able to take more than a single hit from his powerful Infernape.
As shown in Episode 100 of Journeys, Flint actually managed to become a member of the Masters Eight for a period of time but was knocked out off-screen sometime before Ash joined. Despite this, his battles against Trainers like Ash, Cynthia, and Leon make it clear he's very near the top.
9 Raihan Can Challenge The Unbeatable Leon
The Hammerlocke Gym Leader, Raihan, is given a good deal of focus as the rival of Leon, Galar's undefeated Champion. The games have implied that Raihan is on the level of Champions from other regions, and the anime only expanded on this detail.
Raihan spent the majority of Journeys comfortably in the Masters Eight, only knocked out by Ash at the very last moment. He may have fallen just short of the final roster, but knowing his fiery spirit and tricky use of weather-based tactics, this setback won't keep him down for long.
8 Alder Is The Former Unova Champion
Alder was the Champion of the Unova League during the Black & White series. He was characterized as lighthearted and laid-back in the anime, which may explain why he wasn't shown competing in Journeys, but he can get serious and send weaker Trainers packing when the time calls for it.
Alder wasn't shown battling as often as other Champions, but when push comes to shove, he and the Unova Elite Four are just as tough as any other region's. His successor, Iris, quickly rose to the Masters Eight herself, so it only makes sense Alder could do the same if he wanted to.
7 Wallace Can Hold His Own Against Steven Stone
Wallace is currently the Sootopolis Gym Leader, but his true claim to fame in the anime comes from Diamond and Pearl's Wallace Cup arc. He's a world-famous Contest Master and the former Champion of Hoenn, and although the latter title now belongs to Steven Stone, Wallace states himself to be on par with Steven's strength.
Wallace's current character is handled quite a bit differently compared to how he was in Diamond and Pearl, but his accomplishments as a Trainer still stand. Given that Steven was ranked third in the Masters Eight, it's unlikely Wallace would be far behind had he competed himself.
6 Tobias Is Inexplicably Overpowered
Tobias is infamous for handing Ash one of his most crushing and absurd losses in the series - a 6-2 loss after a thrilling victory over Paul at the Sinnoh League. This mysterious Trainer swept Ash's team with Darkrai and Latios, with no explanation on where he got such rare Pokémon to begin with.
As many questions as Tobias raises, it's hard to deny his ludicrous levels of strength. Ash was the only Trainer in Sinnoh to have knocked out even one of his Pokémon, and that raw power would easily give even the strongest Trainers a run for their money.
5 Gladion Has An Affinity For Unique Powerhouses
Although their rivalry is based on mutual respect, Gladion is easily one of Ash's strongest rivals. Several of the five Pokémon he currently owns are exceptionally rare and powerful. As of Journeys, he owns both the Legendary Silvally and the Ultra Beast Nihilego. He also boasts several other powerhouses and unique tricks up his sleeve, such as Z-Moves and the ability to change Silvally's type mid-battle.
Gladion had the upper hand against Ash for the majority of Sun & Moon, cleared every trial of Alola's island challenge, and made it to the finals of the Alola League. Given the fact that he was nearly crowned Alola's first Champion, he's definitely capable of matching top-level opponents.
4 Professor Kukui Has Battled Ash At His Strongest
Professor Kukui was undisputedly the strongest Trainer in Alola prior to the end of Sun & Moon, with his Incineroar regularly defeating three opponents at once in the Battle Royal format. Also, while he doesn't officially own it, he displayed enough skill in battle that the island deity, Tapu Koko, trusted his commands against Ash.
The professor has a brief rematch with Ash in Journeys, where Incineroar battles on par with an even stronger version of Pikachu. Knowing that Ash would go on to challenge the Masters Eight only a few episodes later, this safely places Kukui among the Masters Eight already.
3 Paul Can Mimic & Battle Against Champions
Paul is a Trainer Cynthia took great interest in during Diamond and Pearl, claiming that both he and Ash would someday enter the anime's Champion League. While this didn't quite happen the way she predicted, Paul's return in Journeys emphasizes how much stronger both Trainers have become since then.
Paul's battle with Ash right before the Masters Eight tournament nearly ended in Paul's victory, despite his true intentions being to train Ash rather than defeat him. In addition, despite choosing not to compete, Paul near perfectly replicated the battling styles of several Champions' ace Pokémon. It's clear from this that if he went all out, he could give any Masters Eight member a roaring fight.
2 Palmer Represents The Sinnoh Battle Frontier
Palmer is the father of Ash's rival, Barry. He is also the Tower Tycoon, the Frontier Brain of Sinnoh's Battle Tower. He battled Ash once years ago for a special battle festival in Diamond and Pearl, though Ash's Sinnoh party didn't stand much of a chance against him at the time.
The Sinnoh Battle Frontier was never explored beyond this, but Palmer left a strong impression even in his brief appearances. He's been shown battling against Cynthia, proving he can stand against the best of the best, and his strength along with his connections to Ash and Barry could easily lend itself to top-class battles.
1 Brandon Is The Final Frontier Brain For Good Reason
Brandon, the Pyramid King of the Battle Pyramid, is one of the strongest Trainers Ash ever faced. This mighty Frontier Brain owns a staggering three Legendary titans and was one of the very few Trainers to force Ash into multiple rematches. Not only did Ash need to bring back his oldest partners from Kanto to defeat him, Brandon even limited himself to only one Legendary in their last battle - his newly caught Regice.
Brandon's return in Diamond and Pearl showed his true capabilities when going all out, completely crushing Paul in a 6-on-6 battle without losing a single Pokémon. With the sheer raw power of his Legendary trio, it's unlikely anyone below the highest level is even going to faze him.