Which members of the clergy appear to be corrupt or sinful in The Canterbury Tales?

The Pardoner and the Summoner are the two most corrupt clergymen in Chaucer's Canterbury Tales.  Both these men take advantage of their positions to extort money from those they have vowed to serve.  The Pardoner does exactly as his title suggests:  He offers pardons for sins, but only for a price.  This behavior not only takes advantage of the people he fools, but also it mocks the sacrifice that Christ made so that those same people could obtain a true pardon through Him.  The Pardoner has no shame or remorse for his behavior, saying

But let me briefly make my purpose plain;
I preach for nothing but for greed of gain
And thus I preach against the very vice
I make my living out of - avarice.

He also falsifies relics of the saints which he claims have supernatural powers but are in reality common objects.  He asks exhorbitant amounts of money for what is essentially junk. 

The Summoner also takes advantage of the poor and the uneducated.  He has lackeys who work for him to tell him the gossip about the peasants over whom he has jurisdiction, and he then threatens these peasants with excommunication from the Church unless they bribe him to keep quiet about their indiscretions.  Both these incorrigible men exhibit the views of the Roman Catholic Church that many Brits shared in Chaucer's day.