My vote goes to Miguel Torres. What went wrong in his career? This guy was a killer with an iron chin, but turned into a middle of the pack guy who was winning on points and losing by KO. Wtf?!
He was considered not only the 135lb best, but #2 or 3 in P4P rankings of all time right behind GSP & Silva. 
Id say Crocop. Fedor, Nog, Wand, Chuck, Hughes, etc were all gradual but CC's fall was from OWGP champ to KTFO by GG and never the same.
Of course, when its a Pride guy, we know the reaosons why. If its a Miguel Torres (which is a good example btw), it has to be a real reason.
Lots of new talent arrived on the scene in a short space of time
Frankie Edgar.
His future fights will be vs. killers.
Still a fan of his though.
Fedor was ranked #1 then as soon as he lost he was dropped off the list
XxLiveBaitxX - Frankie Edgar.
His future fights will be vs. killers.
Still a fan of his though.
I dont think arguably beating Benson means he drops much at all.
Shields and BJ 
Probably BJ. He was at the top of a lot of people's lists before the back to back losses to Frankie Edgar.
bj penn
Fedor
Here is a guy, if he cut weight would be fighting at MW or at most LHW, wrecking HW's.
This thread makes me sad...
Joe Stevenson
Mark hominic
Dominick Cruz. He went from being the #1 BW in the world to being taken off most top 10 lists do to inactivity for a year. Which is kind of odd because GSP had been off for nearly 20 months and was never dropped from #1 status
As far as guys that never seem to drop from top 10 no matter what, Vitor is still considered to be one the top MWs in the world.
Despite only being 1-1 in his two fights at MW in the UFC, he's still ranked in the top 10 on all of the top mma websites despite not fighting at MW for the past 17 months. It makes absolutely no sense, the guy's last top 10 win was his KO over Franklin which wasn't even fought at MW and took place in 2009!!!
Wasa-B - Id say Crocop. Fedor, Nog, Wand, Chuck, Hughes, etc were all gradual but CC's fall was from OWGP champ to KTFO by GG and never the same.
Of course, when its a Pride guy, we know the reaosons why. If its a Miguel Torres (which is a good example btw), it has to be a real reason.
This. Winning the Grand Prix to KTFO within a year, and after that his soul was gone.
I think the Hendo KO marked the beginning of Wandy's decline, after that his only win was Jardine until he beat Bisping.
I had Kimbo at number 4 before his tko loss to Big Country
MasterofMartialArts -Wasa-B - Id say Crocop. Fedor, Nog, Wand, Chuck, Hughes, etc were all gradual but CC's fall was from OWGP champ to KTFO by GG and never the same.
Of course, when its a Pride guy, we know the reaosons why. If its a Miguel Torres (which is a good example btw), it has to be a real reason.This. Winning the Grand Prix to KTFO within a year, and after that his soul was gone.
I think the Hendo KO marked the beginning of Wandy's decline, after that his only win was Jardine until he beat Bisping.
With wand I think it was the hunt fight, after that he never looked the same.
But I have to say the most dramatic had to be Fedor then bj, miguel and liddell.
Wasa-B -XxLiveBaitxX - Frankie Edgar.
His future fights will be vs. killers.
Still a fan of his though.
I dont think arguably beating Benson means he drops much at all.
Not so much after the Bendo fights but his upcoming fights he has a really rough road (Aldo, possibly Mendes, KZ,)
55 and 45 are tough classes for him.
Dan Hardy
Faber