Alright, the best I've ever heard was:
I'm a Gamefandan.
The worst ones I've ever heard were:
Why aren't you trying to win?
1. Because you're stupid.
2. Because you're a friend.
3. Because you're a FLAGGOT.
4. Because you fail.
5. All the variations of "your mom".
6. This is a DM?
7. Pie.