Former UFC title challenger Chael Sonnen believes Georges St-Pierre proved he was the greatest fighter of all time after his win over Michael Bisping.

"GSP" became just the fourth fighter to win a title in different divisions after he submitted Bisping on Saturday (4 November) at UFC 217 to become the middleweight champion.

Read more Dana White says UFC 217 did over 1 million buys as Georges St-Pierre becomes middleweight champion

There was a lot of uncertainty about whether St-Pierre (26-2 record in MMA) would be the same fighter, especially after a four-year lay-off and considering that he was moving up a weight class from his old welterweight division.

But the French-Canadian, at times, looked even better during the fight and was never really troubled by the Briton as Sonnen claimed St-Pierre cemented himself as the greatest of all time.

"All the critics were right," Sonnen explained after the fight on ESPN. "They said Georges St-Pierre wouldn't be as fast as he used to be. He wasn't.

"They said Georges St-Pierre wouldn't be able to wrestle like he used to. He didn't. They said Georges St-Pierre would give up muscle against Michael Bisping. He did.

"What everyone learned is the gap between Georges St-Pierre as a mixed martial artist and the rest of the world is too big for anybody else to close.

"That is the greatest fighter we've ever seen – we knew that four years ago but people have short-term memories. They were reminded of that tonight."

There has been a lot of debate as to who the greatest fighter of all time is with the likes of Jon Jones, Demetrious Johnson and Anderson Silva all included in the conversation.

But Sonnen, who has trained with St-Pierre in the past, as well as lost to Jones, believes the debate involves just two fighters and that the new middleweight champion is now the undisputed king of MMA.

"There's only a debate between two guys," Sonnen said. "Is it Jon Jones or is it GSP? One of them broke all of the rules and one of them won two divisions in two different weight classes. That closes the book."

As for what is next for St-Pierre, many believe he will go back to the welterweight division or pursue a super fight with Conor McGregor.

Sonnen, however, thinks he will defend his new belt in a title unification bout with Robert Whittaker, who became the interim middleweight champion back in June.

"I think that [GSP vs McGregor] might be a little bit aggressive," he said. "If you want to know what I think, I don't think Dana [White] is going to make the biggest fight of all time.

"I think he's [GSP] going to stay with the division, stay with his principles, and I think we're going to see [Robert] Whittaker vs Georges St-Pierre."