Cassel sucks, but the vitriol towards him is more due to the regime sticking with him and never trying to upgrade over him. He was never surrounded by a great team which he squandered, he just really wasn't very good.
Grbac should be the easy call here. He choked down the stretch in the '97 playoff loss. He was the guy who got in the way of us keeping Gannon around. He was the one who threw teammates under the bus when things didn't go well. He was the one who ditched this team after the team had been so loyal to him. He was the one who had little love for Chiefs fans. He was the one who, despite being surrounded by a superior team to what we have had the last four years, and despite being the superior talent to Cassel, still only won one division championship during his time (and you could argue Gannon won that down the stretch).
Grbac in a landslide...