Jasmine - She's quiet and sweet. Her Steelix was an interesting challenge in G/S/C the first time I played them.
Whitney - She's whiny, but she's tough. She uses my favorite type of Pokemon, and destroys worlds with her Miltank. (I hated Rollout so much back in the day. ;-; )
Morty - Ghost types are awesome. Also...he's nice to look at. >_>"
Blue - Loved his new gym in HG/SS. He's just lots of fun to battle in general anyway, plus he's super nifty.
Blaine - I always loved going through and doing the quizzes. They were probably one of my favorite things to do in the whole game way back when. I just think he's a neat character in general.