Unfortunately, this depends on what you like and what you're trying to accomplish with the shot. Sometimes I'll use a gridded 7" reflector as a hair light modifier because I want a small, controlled "blast" of light to the hair or shoulder. Other times, I want a more subtle, softer hair and accent light and use a small gridded stripbox. I always use a gridded modifier for my hair light to more precisely control where the light goes, but the modifier itself "depends." My preference is a small gridded strip box (like 9"x36") for individuals and small groups.
Cameras: 1DX, 1D4, 20D, 10D, S90, G2
Lenses: Canon 10-22mm, 16-35mm f2.8L II, 24-70mm f2.8L, 70-200mm f2.8L IS, 300mm f2.8L IS, 200mm f2L IS, 50mm f1.4, 50mm f1.2L, 85mm f1.2L, 1.4x TC, 2x TC, 500D macro, Zeiss 21mm
Lighting: 580EX, Elinchrom 600 RX's, D-Lite 4's, ABR800, 74" Eli Octa, 100cm/70cm DOs, Photoflex Medium Octa and reflectors, PW's, Lastolite Hilite, Newton Di400CR bracket