In order of best to worse imo - Silver, Gary, Barry/Cheren, Belle, Wally, N, Brendan/May.
Silver was the most engaging character out of the lot, and the guy you liked to hate - and had some character development too! Gary as well is someone who I enjoyed to beat and also had a defined personality. Cheren... well he kept on about being 'strong' and all a bit too much for my liking but at least had some personality, and Barry at least was amusing at times and had pretty good theme music too.
Wally was not much of a rival though imo because...you battle him twice (once against a lone Ralts) and basically disappeared for more than half a game. N I just dislike because he felt too much like a stalker to me and his ill-defined team didn't sit that well for me, but the other RSE rivals were just plain boring. They kept being nice when they lost, had no real personality and then gave up in Lilycove without even having their starter fully evolved. And to boot they then try to give you advice on beating the champion. =|