Those who affirmed obligation of the best also disagreed about its scope. Some focused on the servant's religious good; others included religious and worldly benefits; still others framed the issue through reward, facilitating grace, and the conditions of moral responsibility.
Thus the assertion “the best is obligatory” is incomplete until one specifies for which servant, in what respect, and among which possible situations an act is allegedly obligatory.
The Sunni criticism targets the principle of imposing a necessary claim upon Allah, not the existence of genuine benefits for servants.