I would probably pick Karen to as the best. She has this chill, sexy, worldly design. As a trainer she is solid and i happen to really like the Pokemon on her team, and it's a more diverse bunch. Then there's her parting words of wisdom.
Much as I love Hoenn as a region I think Sidney's forgettable. Probably my least favorite dark type trainer. His Pokemon are alright. But I just see a regular punk rocker npc. Dialogue didn't stay with me.
Now Grimsley is one I have difficulty placing. He's striking and maybe the most interesting and developed character of the dark users. He's a cynical, aristocrat with addiction in his past. He has such a stylish ambiance, I just love visiting his Gothic, romantic, old world chamber. He's also got I think by far the most powerful team, but that therein lies the problem. I actually struggle with this trainer to the point that it colors my feelings about him. He's really scummy to verse. He and Clay turned me against the Krookodile line. Some folks struggle with Whitney and hate Milktank for it, I never had issues with them--but Krookorook and Krookodile are different stories.
Nanu kinda interests me. It wasn't the fight against him that stood out in this case, but I like his demeanor, I think his face has character and I like the idea of him as this downtrodden policeman whose plotline ties to other characters. It's interesting always to me to see older characters in a prominent role in the story as well. I feel like he still could be further developed though and that there's untapped potential in him to be seen in future games.
I haven't gotten to meet Piers yet. So I can't give my two cents on him yet, but I am looking forward to the day.