For me, it was Eleni Daniilidou's serve at the Australian Open a few years ago. She was serving to the deuce court and the ball not only went onto the side of ad-court but it was a fault on the far side of the ad-court too!
I can't remember who her opponent was but I think she was just confused. LOL!
Navratalova at Indian Wells this year. Second round, she attempted to volley, however it went about shoulder high to the fence. Come to think of it Kuznetsova wasn't doing so hot either...they got knocked out.
Hingis' infamous smash against Venus certainly ranks as one of the worst smashes she ever hit, so that's definitely the worst shot I've seen at a crucial time by a top player. Some of Serena's drop shots that don't come close to reaching the net, or loop up to the back of the court, have been entertaining.