But, it's NOT the undefeated fighter from Dagestan who is the greatest MMA fighter to ever step in a cage, according to 38-year-old former UFC champ Benson Henderson, who tells TMZ Sports Penn is the G.O.A.T.!

"That's easy, hands down BJ. Not even close. You don't even gotta say his full name, just one word. BJ, and you know who it is. The greatest lightweight all-time, without a doubt, hands down BJ Penn."

Of course, BJ and Khabib -- both now retired -- had very different careers.

Penn started his pro career at the highest level ... in the UFC. He fought everyone from Matt Serra (win, 2002), Matt Hughes (win, 2004), Renzo Gracie (win, 2004), Georges St. Pierre (loss, 2006), Diego Sanchez (win, 2009) and Nick Diaz (loss, 2011). BJ had a rough end to his career ... losing his last 7 fights.

As for Nurmagomedov ... he never lost a single fight. But, Khabib didn't come to the UFC until 2012. However, when he got there ... he literally and figurately kicked everyone's ass.

The Eagle's last 6 fights, all dubs, came against Michael Johnson (2016), Edson Barboza (2017), Al Iaquinta (2018), Conor McGregor (2018), Dustin Poirier (2019) and Justin Gaethje (2020). After the scrap with Justin, Nurmagomdeov walked away from the sport.

We said ... a lot of people believe the Eagle is the GOAT.

Benson's response?

"Out of those 29 and 0 Khabib fought, go look up the records of the 29 people he beat and then go look at the records of guys BJ beat. Go look at the records the guys BJ lost to. Go look at where they fought."

No hesitation there ... for his money, Penn is the best.

And, speaking of money ... we also asked Benson -- a Bellator free agent -- about his future career plans. Does he plan to retire? Fight for another organization like Khabib's Eagle FC?