With all due respect to those who posted, I believe the man was asking about best fletch for shooting Grizzly Broadheads. The Griz is specially ground to create a torque or spinning motion upon impact that is best suited for right wing feathers. (4) four inch Right wing feathers mounted on a helical twist will do you quite well. The 5 1/2 inch banana cut feathers will get you spinning quicker but ultimatley will create a lot of drag, thereby effecting your down range velocity in a negative way.
Short answer: (4) four inch feathers, helical twist is what you want for the Grizzly Broadhead.
FYI, I've been slotting those Grizzly's w/ a dremel tool and the Magnus Bleeders fit right in there nicely for those that want the option of converting to a 4 blade Grizzly.