There have only been eight of them -- UN Secretary Generals, that is. CFR has posted a timeline of UN Secretary Generals which includes brief biographies of all the men who have held the top job in the world body. It's clear from his bio that the present Sec-Gen is not to be counted among the high-achievers at the post.
In the wake of the Burma cyclone aid fiasco, a retired senior UN diplomat told me he thought Ban Ki-moon -- who has held the job since 2007 -- had "six months to a year" to convince people that he was really up to the task. He said the clock was running out on Ban Ki-moon.