Lee Mason had a shocker today in the Villa vs United game and I've seen him completely lose control of games in the past, but I'll say the undisputed worst ref is Phil Dowd, and he's held that title for a few seasons now.
He's just a shocking ref. I'll never forget the time when Vidic fouled Agbonlahor in the Carling Cup Final a few years ago and it was clearly a straight red and a penalty, but Dowd refused to send Vidic off as it was only 4 minutes in and a red card at this stage "would have affected the game", so he just awarded the penalty and let Vidic off. Like, what the fuck? He also struggles to keep up with play as well, I'm not sure how he's still able to referee in the top flight.
Neil Swarbrick is quite bad too. I don't think I've seen him have a good game, actually.