I like Blaine from FR/LG and Norman the best from R/S/E, though not neccessarily as opponents. I always thought that the trivia questions were fun in the Cinnabar Gym (even if they were easy), and I wish we would have gotten a little more backstory about Blaine and Mr. Fuji.
I liked that Norman was introduced as your father in R/S/E, and he is still the only one that we know of throughout all the games.