Best how?
I think Silver had the best in game character arc and antagonist relationship. He transforms from a smash and grab on the streets punk into a function member of society while you teach him the true power of love and hard work.
Barry has the best fantheory arc. To me, his nervous energy eventually becoming the dedication for an ace trainer level team is enough to launch him over Silver and into second place.
But really the best rival is the rival who always shows up just at the end of a long tunnel, or in the middle of you saving the day in hopes of stealing the glory you rightly deserve. Just when you think you're champion, he's there to try to stop you. Really the best rival is Red .
