It takes a lot to be considered one of the greatest of all time in anything, but in a sport like MMA, you need both talent and luck on your side.
Fedor Emelianenko went almost 10 full years without a loss, but has suffered three in a row.
Anderson Silva has looked unbeatable for almost six years, but he still has a few losses from early in his career.
Hell, once upon a time Matt Serra won a reality show and took out Georges St-Pierre.
My point is the sport changes too quickly and is too unpredictable for anyone to be perfect. While none of the fighters on this list are perfect, they are the best this young sport has given us.
As always with a list like this, some of you will disagree to the point where you want to beat me to death, but instead, please just leave a comment.