Definitely have to be the gen 6 rivals.
Everyone complains about May/Brendan and I agree that fight sucks, but that's mostly due to poor game design. Early in the game they put up a good fight.
Hugh is pretty standard by rival standards. I don't see why people hate him as he's just basic. He gets the job done.
I actually like Hau. I like how's he's friendly to you and not everyone in the planet wants to battle you seriously. Plus, I saw Gladion as our proper rival anyway considering he had the Sivally that changed depending on your starter.
The gen 6 rivals are complete trash. I'll be fair to the "friend" characters as they aren't technically rivals. They still suck as they barely put up a fight. But I'll only focus on Selena/Calem. They suck. They suck hard. They barely catch up in levels to you, they're the most boring people imaginable, the writers try to force this rivalry on you, but it comes off as so forced. It's awful. Every time I fought them, I was so disatisfied and that grew with my recent playthrough of Y. To make matters worse the character gets worse every time you see them. They get more frustrated with you and are clearly mad about losing, but....they never address this point? Ever? And then your rival acts so nice to you after the climax of the story...what? It's just terrible.
Say what you want about the other character's but I'd rather have bland writing that writing that makes me want to pull my hair out.