My favorite BW rival has to be Kenyan, if the writers plan on keeping him around at the end of the Don Battle.
What I love about Kenyan's character the most is the simplicty of its execution. To put it frankly, Kenyan is only present to battle certain trainers to the best of his ability. He doesn't drag down the mood of an episode by picking fights with specific characters, acting like a jerk just for the fun of it, or being ineptly overactive, immature, or naive. I always loved the down-to-earth rivals, simply because they were overshadowed by the other recurring characters who made an effort to speak out boldly and loudly every time they appear. They are the relative underdogs in this anime series, but are still powerful and respectable battlers in their own rights.
I always disliked how the writers put emotional tension first when it comes to introducing specific rival characters. Langley, for example, was entirely built around her dislike of Dragon-type pokemon in general, which stemmed from a loss against a Dragon-type trainer in the Dragon Village. That soon led to a confrontation with Iris (who loves and trains Dragon-types). This builds up a lot of unnecessary tension that fails to go anywhere. Iris and Langley will eventually fight each other, and it may be possible that these two characters may still despise each other by the time this series ends.
Burgundy is also yet another example. She may be eccentric in her own right, but I always pegged her out to be the inferior and female version of Cilan. As a younger trainer, Burgundy displays many of youth's foulest qualities such as impulsiveness and a narrow minded way of thinking. Burgundy will always be treated as the inferior and undeveloped connoisseur in my eyes unless the writers do plan to give the character a noticeable sense of growth in her respective field. I want to see what the writers are planning to do to convey that sense of growth, but right now I am not impressed by her presence in the anime.
I am sorry to say, but I am not fascinated by Bianca's presence in the anime, either. In my opinion, Bianca serves as nothing more than an inexperienced trainer who hopelessly requires additional assistance with other real life functions. Bianca's ineptitude can best be explained by a combination of her anxiety and excitement. The writers are not impressing me by introducing a "secondary rival" character who is only present to cause trouble for everybody else. My impression of the character cannot be helped since the anime focused on a similar rival that functioned in nearly the exact same ways almost a year ago. Unlike Barry, Bianca lacks both that experience and expertise in battling, as well as a solid goal or defining point in which the character can improve over time.
Bianca simply wants to collect as many gym badges as she can, and rushes off-screen in the exact same fashion that she had appeared on-screen in the same episode. I cannot take Bianca's endeavors with a serious face, yet.
With Kenyan's presence, you get the impression that a pokemon trainer does not need a petty reason to justify battling against others with everything that he or she has. Even though Kenyan has the capacity to display emotion (such as being visibly bothered that some characters fail to pronounce his name correctly), he does not let that become the basis of his decision to battle specific trainers. Kenyan shrugging this off most of the time is a testimony of his maturity.
Kenyan certainly stands out the most out of all of the BW rivals, just because he does not make a big promotion every time he makes an appearance. Furthermore, his presence in the anime will not be limited to picking fights with specific trainers out of spite or a desire to best them. Kenyan will battle anybody who will give him that good fight. I will certainly be rooting for this underdog as this series continues.