All are good in their own right. Suicides or mystic dispersion in your arsenal? Do you already have a Herc, or not - cause if not, CGR is gonna just smash things even better at R4. I would only consider Maw an r4 candidate if you're a huge fan and running high mystic dispersion most of the time. I have an R2 ihulk and he's growing on me, but he does have a finicky play stye. Penni is the easiest to use and has unique features. I don't have knull, but if running suicides and he gets duped, he's going to be fun for offense and defense.Without knowing what else you already have in your roster, hard to say. Go with your gut. More r4s will com so pick the one you're going to enjoy the most now.