Bill Walker. If Walker's in the game, he might well be playing D on Artest. KG's too smart to get into a fight. Powe, he's tough as nails but he's never come close (that I can recall) to getting into a fight on the court. Perk - I could maybe see him getting involved, but he's learned to control his emotions a bit (I remember him almost getting into a fight as we were being beaten by Indiana in the playoffs his rookie year).
BBD's a lover, not a fighter.
Walker, though... he seems like he might be a bit of a loose cannon. Not necessarily bad, but he seems like a VERY emotional player that's going to have to learn how to channel that (ideally, he channels his aggression the same way KG channels his). He kind of reminds me of Tony Allen during TA's rookie year; a wee bit out of control, you don't really want to take your eyes off him.
Plus, he's already gotten mixed up with them, McGrady threw that shoulder at him, and we know he's not going to back down.