UFC's 'Wonderboy' Thompson Says Cyborg Would Beat Rousey

She might be the best ever ... but Ronda Rousey would LOSE if she does get in the Octagon with Cris "Cyborg" Justino ... so says UFC star Stephen "Wonderboy" Thompson.

TMZ Sports broke the story ... Rousey told Dana White she WANTS to square off with Cyborg -- in what could be the biggest female MMA fight of all-time.

But Thompson says Rousey should think twice -- because Cyborg is no punk ... and when it comes down to it, he thinks she would crush Ronda.

Thompson explains why -- check out the clip.