May/Brendan had no reason to be in the games as rivals whatsoever, especially how Wally was your other rival who actually had a story with character growth, and you battled him for him to prove that he had grown not only as a trainer, but as a person (not just for fun like with May/Brendan). On top of that, he actually used the final evolved form of his starting Pokemon.
I'm not sure what's worse: that Brendan/May, lovely though they were, didn't even bother trying, or that Silverdid try, but was an utter failure. He started out completely unlikeable, then ended up dithering on whether or not he wanted to continue being a douchebag, thus ending his rival tenure as a conflicted nothing.
Barry at least had spirit, but he was still an irritating little ****. Haven't played 4th Gen yet, so I can't comment on that lot.
Really, the only good rival I've come across was Blue. And my goodness, what a rival he was! He was fantastic. An absolute masterclass in being a great rival. He was a jackass, but one who actually walked the walk, so you actually loved to hate him - perhaps even grudgingly respected him. Everything you did, everything you wanted to do, everything you dreamed of becoming: he got there first and you had to fight him for it. He became Pokémon Champion before your. Your ultimate goal, your ultimate dream: he achieved it before you and you had to take it from him. What a guy. What a magnificent bastard.