My favourite Rival has to be Silver- he is the most darkest character any of the games ever had, and he never has any stupid lines or gags. Next would be Jun/Barry/Pearl, because he is not dark like Silver, but his seriousness about battling comes close.
Next would be Blue/Gary, coz his wise-cracks and cockiness are annoying, and last would be Sapphire/May, because she is the most frustratingly annoying Rival in any of the games. (I don't really consider Wally as a Rival, but i like him better than Blue/Gary).