Best rule in Pathfinder: Rule 0. The GM is empowered to change the game in any way she sees fit. As a GM, ensure that the purpose of such changes is to make the game more fun for players and GM alike. Remember, fun is the entire point of the game.
Worst rule in Pathfinder: Alignment. It's a relic of older RPGs, and it should have been tossed well before THAC0 and system shock rolls. I have yet to see a single game of Pathfinder or its predecessors that was improved by alignment, and I've seen plenty that have been the worse for it. Consign it to the trash heap of game design history.