Favorite Rival is Silver. Blue and Gladion were cool too, a close second and third place. I just loved beating their smug asses and looked forward to every encounter with them.
Barry, Hugh, and Hau were all right, but not quite as fun to kick their asses.
Cheren, May, Wally, and all those XY fuckers were a bunch of lamewads, but Wally was slightly less of a lamewad than the others. Shauna at least had the slightly redeeming quality of being cute. I felt like a bully battling these guys, especially Shauna.
I don't know if N counts as a rival (more like anti hero I guess?) but he kinda sucked too, BW's plot was kind of a dumpster fire in general, but BW2s was acceptable.
I want a rival that gets desperate to beat you and goes full crazy-fucker shonen anime and learns some kind of forbidden evil technique to try and fight you. He's basically cheating. Then he's super fucking salty when you beat him, even when he cheated! And throughout the game he just cheats more and more and gets madder and madder the more you beat him. Eventually he decides to stop cheating and is actually tougher when he follows the damn rules. Would be a fun direction to take them.