Its close between Grimmjow and Ulquiorra. I like grimmjow because he will do whatever it takes to win kinda like kenpachi, who is my favorite shinigami. Ulquiorra however is more calculating and is able to analyze his opponent better and look for an opening.
I gotta go with Grimmjow though because he's more aggressive.
"Every man shall reap what he has sown, from the highest lord to the lowest gutter rat. And some will lose more than the tips off their fingers, I promise you. They have made my kingdom bleed, and I do not forget that"