Since Federer took over as world #1 in 2004, only him, Nadal and Djokovic have held it and none of those guys would qualify as the worst since 1990. I pick 1990 because it was the start of a decade that saw many switches at the #1 ranking, some that were very short. My short list for worst #1 ranked players: Marcelo Ríos-Never won a Slam. Reached the Aussie Final and five other Quarter-final appearances, two more in Australia. He held the ranking twice for a total of 6 weeks. Thomas Muster-We all know the story of his 1 week reign after winning the French, his only Slam. He made 3 quarters at the U.S. and 2 semifinals in Australia but several early round exits including his strongest surface and one of the worst top player on grass ever. Carlos Moya-A Final appearance at the Aussie in 97 and then a win at the 98 French and semifinal appearance in United States with not much after.