There have been some shockers over the years who should've got nowhere near umpiring at international level (e.g. Rudi Koertzen, Billy Bowden, Asoka de Silva etc)
However, since the ICC elite panel of umpires was formed in 2002 (and therefore, neutral umpires for test matches), Russell Tiffin is the umpire that I can remember who would give Joel Wilson a run for his money for worst umpire. Still remember the 2002/03 summer just like yesterday. Tiffin made numerous elementary blunders in both the Ashes and ODI tri-series.
My favourite howler from Tiffin was in an ODI tournament in Kenya about 3 months prior to the 2002/03 summer. In an Australia vs Pakistan ODI match, Jason Gillespie bowled a Yorker that went between Abdul Razzaq's legs when Razzaq was trying to dodge it...and Tiffin called a WIDE!!!