Winona-- like everybody else has said by now, she has good pokemon (and she's pretty tough), love her gym's design (especially in Emerald), and she has a cool name!
Flannery--She really wasn't that hard for me since I had a swampert. But she did have a great team (somewhat annoying torkoal), and her gym (in Emerald) was kind of a challenge for me to get through.
Wallace--Imo, Juan was alright, he was a good battler but he still can't top Wallace.