Karen is hands down my favourite.
The Johto League was the first one I ever faced, and from the moment she addressed me: "I love dark-type Pokémon. I find their wild, tough image to be so appealing. Think you can take them? Just try to entertain me. Let's go," it made a huge impression on me and made me determined to impress her with my battling prowess lol xD So when she eventually said: "Truly skilled trainers should try to win with their favorites. I like your style. You understand what's important." I was so stoked lol xD Also I thought it was super cool that I had an Umbreon on my team and she had one as well, not to mention... her Houndoom!! The first time I ever saw that Pokémon was when I battled hers and it became an immediate favourite then and there :) Her whole demeanour really stood out to me, so calm, cool and collected which was a stark contrast to the other members :)
I also like Drasna and Sidney for their personalities :)