I haven't tried Flat Patties, but as a huge fan of the West Coast style burger I'm looking forward to trying it. I think Green St.'s double cheeseburger is a strong competitor for that style as well.
On the East Coast style, I'm glad to see someone agree that R.F. O'Sullivan's is totally over-rated.
Confusingly, his rankings seem to go from 1 - best to 10 - worst.