Blake Griffin tells TMZ Sports that Zion Williamson can "100 percent" be the NBA's all-time best dunker ... saying of the former Duke star, "He's going to be a monster."

Of course, Zion has been legendarily rattling rims since he was a high school prodigy ... and now that he's on his way to the NBA, we had to ask Blake if Z can be one of the best dunkers ever.

Blake's opinion? He says there's no doubt in his mind Zion can be up there with Michael Jordan and Dominique Wilkins, telling us, "100 percent. 100 percent."

"If he keeps playing like he is, he's going to be a monster."