I couldn't pin down one line, but this is a couple of my favourite Elaine lines.
Can't remember which episode it is, but Elaine, George and Jerry are in Jerry's apartment. George goofily comments to Elaine that he had always hoped that Jerry and her would get back together. She glares at him and gives a beautifully delivered line:
"That's because you're an idiot."
She says it so perfectly, you expect George to just wither up and die.
Another great George-Elaine exchange is when George is wearing his toupee which Elaine hates. She grabs it off his head and runs to the window and says:
"I don't like this, and this is what I'm doing with it!"
and she then proceeds to throw it out the window. She is so wound up and desperate about that toupee. It's fantastic.
Cheers, Pete