Over the course of an entire career I would definitely have to say Beckham. People are still clamouring for him to be starting for England, but he's extremely slow, one-footed and looks totally lost without Neville to pass it back to when he gets to an opposing player!
However, Trezeguet definitely gives him a run for his money. At the time he was rated higher than Henry and getting into the France starting XI ahead of him, he was always a disappointment to watch, as he never seemed to do anything. Bit like C. Ronaldo, great at ordinary club level, but whenever the pressure is on or he's against good opposition he disappears off the face of the planet!