The G/S/C rival was by far my fave, as other have mentioned before, he has a lot of development in the game and is actually interesting. And the fact that he stole his first pokémon really makes him stand out next to the rest of the rivals.
Gary comes in after him though, kind of annoying, but it was kind of made up for it in G/S/C when he became a gym leader. Honestly, this was the only time we ever saw any development in his character.
R/S/E's Rival? I just wasn't feeling it that game, felt really tacked on.
DP had.. 3 rivals? Your opposite gender counterpart, Wally, and uh.. that one other kid >_> forgot the name. The 'assistant' wasn't much of a rival, you didn't really fight him much and he wasn't even that hard. Wally well.. you more 'trained' him than really rivaled against him, and the other guy, well I liked his personality at least but I don't recall him being a real challenge.