What was Luther's biggest criticism against the Catholic Church?

Luther railed bitterly against what he saw as the opulence, immorality, and rampant corruption of the Catholic Church. He genuinely believed the Pope to be the Anti-Christ, distorting the original message of the Gospel to consolidate the wealth and worldly power of the curia, or papal court.
In his increasingly vicious attacks upon the Catholic Church, Luther was particularly scathing of the concept of indulgences. These were certificates which enabled believers to buy remission of sins or a reduction in divine punishment for themselves and their loved ones. Luther, as with other Protestants, regarded the sale of indulgences as a wholly pernicious practice, a perversion of the Reformation doctrine of sola fide whereby any remission of sins was entirely an act of God's grace. Human beings had no part to play in the matter whatsoever, whether by the purchase of indulgences or any other method.
Though Luther was of course highly critical of the Catholic Church for its enormous wealth and worldly pomp, his main grievance against it was theological. What mattered to him, more than anything else, was the Word of God as revealed in the Bible. And as far as Luther and the other Protestant reformers were concerned, the Catholic Church was guilty of the willful distortion and subversion of God's Word.