The most hated character within the show, by the other characters, was most likely Ralph Cifaretto. It was apparent that the other members of the Soprano family didn't like him, but he was tolerated because he excelled at being an earner, and in the Mob, that's the main goal, earning money for the family. He made Tony a lot of money, even though Tony clearly hated the man. In the end, Tony killed him with his bare hands, and that was that for Ralph. Some memorable quotes about Ralph:

Tony: If Ralph was drowning, I'd throw him a cinder block.

Albert Barese: Ralph? I wouldn't piss on the guy if he was on fire.

There are several other memorable quotes about Ralph, but those two are the first that come to mind.