Had to say Burgh. The cool artist that constantly disappears from his gym. He became my favorite gym leader in b/w due to his roll in the story. Him, Clay, and Iris played the big gym leader rolls in the story and idk.. Burgh was just the coolest one.
The least favorite is Clay, though. Just because he is the only one that ever takes more than one try for me to beat with vigorous training in between attempts.