Every new Pokémon adventure starts with the selection of your starter pokémon and the introduction of your rival. These characters started as competitive challengers who looked to take you down to make themselves better but have morphed in recent generations to be close friends who happen to want to complete with you.
This has caused a divide in the community as to which rivals are the best throughout the generations. Let's now rank every rival trainer from all Pokémon generations.
10 Hop
Hop comes in at #10 on the list simply because trainers still don't know much about him. The only interesting bit of information about Hop is that his brother is the current Pokémon Champion of the Galar region and he's training to be just like his brother. It's unclear what this relationship will result in and fans are hoping there's some kind of twist or surprise in the narrative. Will Hop end up being a close friend who's by your side? Probably, but there's still a chance he gets pulled to the dark side.
9 Trace
If you didn't happen to give Let's GO Pikachu & Eevee a chance then you're likely unfamiliar with Trace. He's meant to convey the same role as the rivals from the original Kanto adventures. The biggest problem with Trace and why he's so low on the rankings is because he's just not very difficult. This is more of a problem with the Let's GO games than it is Trace, but this was a purposeful design decision as these games were meant to be points of entry for younger and/or newer Pokémon trainers.
8 Barry
At first glance, Barry looks like a palette swap of Ash Ketchum, but this rival from Pokémon Diamond & Pearl is much more than that. The blonde-haired kid is super impulsive and very hyperactive. He's the type of rival who is a joke and an afterthought up until the final encounter where is growth is obvious and apparent. Barry ends up having some of the highest level pokémon of any rival which is a testament to his commitment and tenacity as a pokémon trainer in the Sinnoh region.
7 Serena/Calem
Serena and Calem from Pokémon X & Y are the second time in the series where the rival will be determined by the sex the player chooses at the beginning of the game. Calem will be the rival if the player is female and Serena will be the rival if the player is male. Outside of name and appearance, their function in the story is the same as their lineup of pokémon are identical. Their full roster isn't too intense, but it does feature one of the original 3 evolutions for Eevee which is pretty neat.
6 Cheren
Cheren falls into the category of the studious and impatient trainer who just wants to excel. He first appears as the rival in Pokémon Black & White and later becomes a gym leader in the sequels.
This type of growth is great to see as it shows that his attitude as a rival paid off for him as he accomplished his goals and became a gym leader. Far too often the rivals have personal goals they never seem to fully achieve, but trainers can see Cheren reach his full potential in the games.
5 May/Brendan
May and Brendan are the rivals trainers can face in Pokémon Ruby & Sapphire and the one you'll face will be the opposite sex of your character. They sport identical rosters of pokémon so there isn't much difference between the two when it comes to battling. Their lineup is solid as whichever starter they're left with is accompanied by 2 other pokémon of the other starter typings as well as an electric and flying-type pokémon. Having a well-round team as they do makes trainers focus on the rival battles.
4 Hugh
When it comes to rivals it's all about preference and for most trainers they'd likely lean towards having a rival who's serious and focused, which is exactly what Hugh is. He's the rival in the sequel games to Pokémon Black & White and very much feels like a cool anime character. He happens to sport a very strong and diverse team which means he'll likely always feel like a decent challenge at the very least for trainers. He also happens to be the rival trainers face the most in any generation at a total of 7.
3 Hau
Hau is an interesting case because he's not only a prime example of how the rival role has morphed over the years, but he happens to be the only rival who selects a type disadvantage when it comes to selecting starter pokémon at the beginning of the game.
In the Ultra versions of both Sun and Moon, he becomes way more interesting as he achieves his goal of becoming champion at the end of the game and sports a full team of 6 pokémon. Hau was always way too nice to be labeled a rival.
2 Silver
Silver is easily one of the best rivals in the series and that mostly has to do with his story. He'll quickly come across as a very serious individual who sports brash cruelty at times. Trainers come to find out that he suffers from abandonment issues due to his dad leaving. It doesn't help that his dad happens to be the head of Team Rocket, Giovanni. He uses this to help fuel him as a trainer so he can become better than his dad ever was and show him he didn't need him after all.
1 Blue
Blue doesn't shoot to the top of this list simply because he was the rival of the beloved original games. He happens to encapsulate what most fans are looking for in a rival during their Pokémon adventures. Blue was always a little mean, but he was simply trying to challenge himself and make both of you better trainers for it. As the generations have gone on the rivals in these games have transitioned from competitive rivals to best friends that you battle sometimes. Blue understood the value of being a true rival.