Wow, I'm actually quite surprised by the lack of Silver supporters in this thread.
Very surprised...
In all honesty, I have to say Silver was my favorite rival. He was the true epitome of a bad guy. He stole his first Pokémon, thought of the little creatures as tools, and only had aspirations to make them stronger. It was up to you to set him straight and teach him to respect his Pokémon. It is for that reason that I liked having him as a rival.
Gary, May, and the rival from the fourth generation weren't quite up to snuff with me. Sure, if was pleasurable to defeat them at every chance I received, but there really wasn't any satisfaction of "ridding the world of its evil."