To put it bluntly, Aizen only cares about himself, and views everyone else, allies and enemies alike, as pawns to further his own selfish goals, though some of these twisted relationships have a level of uniqueness.
Aizen ruled the Espada both through fear and his own remarkable charisma. However, he viewed them like he viewed everyone else: as pawns. In the end, Aizen's callousness, coupled with Ben Tennyson's charisma, caused a number of the Espada, and even some of the Numeros, to defect.
Tosen was fervently loyal to Aizen and would obey his every command without question. Aizen, in turn, manipulated Tosen's warped views of "justice" to make him a willing pawn in the insidious Shinigami's ambitions, feeling nothing towards him and being fully willing to callously dispose of him once he had outlived his usefulness.
Aizen was intrigued by Ichigo, but would ultimately dismiss him as an overall threat to his plans over Ben.
Outwardly, Gin seemed to be a loyal subordinate to Aizen, but in reality, Gin despised him for nearly killing his friend, Rangiku Matsumoto, and planned to kill him. Aizen apparently did not anticipate this, or did not show any signs of it, and was completely shocked when Gin fatally stabbed him.
Aizen feared Urahara's intellect, which he acknowledged as greater than his own. However, he remained confident that he would outsmart Urahara.
Aizen rules the Espada through fear, and cares nothing for them.