You know, although I haven't played BW, I'll go with Hilda. I really like her design as it's modern and trendy. I feel if I was a Pokemon trainer, I'd dress almost exactly like her. Her outfit is comfortable and her boots and hat give her that extra boost in the confidence look. Like, "Yeah! I'm ready for this adventure!". You also have those lightweight clothes for comfort. Nothing too restricting in the legs for flexibility. And of course, that sweet vest and athletic wristband that just make her really pop and give off an adventurer feeling.
Of course, I'm really liking May's design in ORAS too. I think she's cute, but overall in terms of character design I'd say Hilda takes the cake. Granted, this thread is probably for more than just design but since I haven't played the game or read the comics, I can't say much about her character. I'm sure she's sweet though. ^-^