Least Favorite in-Game Rival?

[Hikamaru]

Can't really blame them; it was worth a try for Game Freak to bring in multiple rivals, and while that had potential for the most part, it all fell rather flat. u_u

The bad reception of the Kalos Group goes to show quality is better than quantity, and I guess Game Freak learned from it by reverting back to one rival for Sun & Moon.

 

[Rivvon]

I'm not gonna count the XY group in this because they're meant to be your "friends" and not rivals. But if they were considered as rivals I'd probably name them as my least favorite because I never enjoyed when they showed up. Ever. XY's actual rival, Calem, was disappointing in the character development department but still bearable so no one from XY takes my pick.

In a similar vein, (ORAS) Brendan isn't quite your rival but if he was ohhhhh boy he'd be my #1 pick I absolutely cannot stand him.

I know this is a really unpopular opinion but I guess I'd have to go with your gen 1 rival (Blue). Silver is awesome and has great character development, Wally is great (especially in ORAS), I like Barry because I think he and his personality are pretty cute, and all the rivals in gen 5 are spectacular. Blue is just kinda mean and doesn't have any sort of growth throughout the game--I mean, it works, because that makes you want to defeat him even more once you make it to the Pokémon League, but in the end he feels kinda bland when he has no development and Silver filled the whole "nasty rival" role better and had character development on top of that.

 

[Altairis]

The XY rivals were pretty disappointing. I found them to be pretty bland, especially Calem/Serena. They seemed soooo generic, and we had so many of them, so it seemed almost worse. They didn't really seem to change at all or have a special personality that wasn't so basic, so all of them are my least favorites equally :(

 

[Drayton]

Kalos because way too many rivals that participate in battle less exclude Shauna/Caleum also both of them are just isn't good and fun at all, just wasted potential having 2 or 1 is just fine, but more than 2 is just WHUT! Before ORAS, Brendan/May just went too much overboard and not fun, but in ORAS things get twisty and exciting also Wally developed a lot in ORAS than RSE

 

[Del]

Bianca was so digusting!!!

 

[Lapis]

The XY rivals. As many other people have said, these characters just didn't really seem to have a personality. :/