This argument is getting so heated, lol. I'm going to put in an opinion of my own.
- Bianca wins the most improved rival in my books.
Honestly speaking, like what joltik said, Bianca was the only BW rival that actually developed. She starts of as a klutz and complete bs in battles. She beats Trip, Ash's main rival and Georgia. She placed 64 in her first league, pretty good in the anime world, considering how Trip got placed at 128. She KO'd Kot's Samurott, who haxed Escavelier thanks to plot critical hit. She
falls in loveinteracts with Virgil mainly and Ash's other rivals. Considering how she was more of a klutz than Ash from the beginning of OS, I say she had really improve. The only one that has came close is Georgia, who just caught a lot of Pokemon offscreen. Kotetsu and Virgil are too late in the series, while Burgundy just gets the writer's hate. Trip... his only improvement was evolving his one-man team Serperior. But yeah.