I agree. I miss the days of Blue & Silver. They were such antithesis of the main characters that you always wanted to one up them.
Brendan/May were also good, especially in the remakes were you got the relationship aspect for a real change. Sick boy Wally becoming a trainer was also a nice side story.
Barry was awesome in Sinnoh. The best friend approach but still a fierce rival, also felt like a real person.
Cheren was pooh, but let's be real: N made B/W. 5th Gen was really when the rivals started to diminish.
Serena (and friends) was horrible in Galar; she was a much better suited for the Anime. Then again, Galar as a whole wasn't the best.
Hau was crud too, but at least you had Lillie and Gladion. If not for them as supporting characters, along with the first ever "Who's the real bad guys?" approach, S/M would've been a bust.
Hop wasn't that great either. The little brother plot was different, but he basically was a male version of Bianca from Unova. You know, never good enough so I'll just become a Professor's Assistant. Bore.