Bryan Danielson -- known to WWE fans as Daniel Bryan -- is taking part in what could be his last wrestling match ever ... and if that is the case -- what will his legacy be?

Well, according to Nikki Garcia -- whose twin sister, Brie, is married to Danielson -- he'll be remembered as one of the G.O.A.T's!!!

TMZ Sports caught up with the former WWE Superstar out in L.A. this week and got her thoughts on her brother-in-law's Title Vs. Career match this Sunday at AEW All In ... which could mark the end of his legendary career.

If it is the end ... Garcia says "he is one of the greatest wrestlers in wrestling history."

"He truly is one of the most selfless people in the industry," she said. "I've learned a lot from him in my career. Anyone that he was in a story with, he was always so giving."

While she couldn't name one particular wrestler Danielson could pass the baton to if he loses ... she said there are a ton of talented individuals in AEW that would be more than worthy.

Shifting to the world of Nikki ... it was announced in June she will be a part of the Season Three cast of Peacock's competition series, "The Traitors." While she has done many projects over her career ... she says the show was unlike anything else.

"I mean full on," she said. "Especially when you're 'Survivor' people or 'Big Brother.' I mean, it's totally a different mindset. Like I'm used to real competition, that's like totally different kind of [competition]."