Fedor won the title in 2003, and held on to it until the company ended in 2006. Throughout that time he had 15 fights, and won every single won of them. The championship wasn't defended in all of them, but he stayed active nonetheless (unlike UFC Champions, who only fight twice a year if we're lucky, with the exception of Jon Jones who has been a great champion to this point), and he kept winning.
And when he fought Big Nog (all 3 times) and Cro Cop, they were ranked as the 2nd best Heavyweight in the World. Everyone else he faced, while not necessarily the cream of the crop, were still coming off wins. And to make it more impressive, the two dominant Heavyweight Champions in the UFC during Fedor's reign, Fedor would go on to destroy the both of them just as they left the UFC to show that he truly was the greatest Heavyweight around from 2000-2009. That's close to a ten year dominant reign. Think about that for a second. And to top it off, not one of those fights ended up being boring or a letdown. With Fedor you ALWAYS got a vastly entertaining fight, and a top notch performance. There will never be another run like it.