A screwdriver is most effective when the width of the head of the driver matches the width of the slot in the head of the screw. If you are going to buy just one of each, I recommend a number two Phillips head and a quarter-inch flat blade. However, I recommend that you get at least two sizes of each. In fact, you should really have a complete set of screwdrivers in your workshop to match the variety of jobs that occur around the house.