Really, it depends on what you consider a Watcher to be. Giles was certainly the most helpful to Buffy in that he actually took part in patrolling the hellmouth, fighting demons, and such, but from the few times that the other Watchers visited from England, I got the distinct feeling that they were not happy with the way that he was fulfilling his duties. I think that the traditional Watcher's duty, once the Potential has been trained and Called is to do exactly what his title implies: watch. At best, he should fill the same role as an athlete's coach. He trains and directs, but never actually participates in the action. From that point on, he basically just chronicles the life, career, and death of Slayer X, maintaining as much detatchment as possible and doing everything that he can to keep her activities covert.
By that standard, despite his goofiness, which was almost certainly due primarily to his youth and lack of experience, Wesley probably fulfilled the actual duties of Watcher better than Giles did, because he maintained his detachment more.
