I know he had a solid career before Seinfeld, but crazy that at a later point in life, he created two iconic (well, one iconic, and one amazing) supporting characters like he did.
I talked in another thread about supporting characters and how they are best when they (duh) support the show. A Frank Costanza show would never have worked. But man, having him in bits and pieces on Seinfeld (and King of Queens) was brilliance. Never before or since have characters like Frank and Estelle given insight in to how another character, George, is the way he is.