RBY: Gary was a great rival, because there was a true rivalry between him and Ash (or Red, if this is for the video games). You actually cared about beating him and wiping that smug look off of his face lol
GSC: Never really played GSC much, can't comment.
RSE: These guys sucked. You know nothing about them. They just pop up in the most inconvenient places, all happy and cheery, then lose to you. They don't have much personality either, really. Boo.
DPP: Can prove to be pretty good competition at times, but again like RSE... meh.
My vote goes to Gary.