Best: Wendy's. They've tried to colonise the UK twice and failed twice, but whenever I'm in the States I make sure to have at least one Wendy burger while I'm there. In the UK the best is the East Sussex hipster burger chain Half-Man Half-Burger. Their rosemary fries are to die for.
Worst. Wimpy - a hopefully now-defunct 1960s British attempt at a burger bar, only with shades of Lyons Corner house about it so it had waitress service, sometimes in the same hour you ordered your meal in. Perhaps it was the toenail that I found in my fries that put me off them.