For sheer bluntness, here's one I tend to favor:

"You're hot and all, but your dumber then the dirt I'm standing on. Let's just call it a miracle we lasted this long. Later."

Works every time, because they preen at the compliment, don't get the insult, and the preening offsets the sadness.

Note: That one only works on good looking yet stupid women. I have reports that it doesn't work on men.
I KNOW it doesn't work on half-intelligent women.

Oh, and here's one for the girl you don't want to go face to face with:

Her: "What's up?"
You: "Nothing. Let's meet for a talk. I'll bring my helmet and cup."
Her: "You want to break up with me??"
You: "Uhh.... how'd ya guess?"

By that point, she should have hung up. Problem solved.