I think it would differ greatly whether I ran into them on the streets vs in the ring.
If I was confronted by Ali, Oscar, Trinidad, Mayweather, Leonard, GGG, even Wladamir etc on the streets and didn't know any better I would laugh them off. (an exaggeration as I am a bit of a pussy in my older years and get scared by loud noises)
Where as guys like Mayorga, Barkley, Foreman, Duran, Liston, Hearns would make me poo my pants a little.
They'd all kick my ass either way with the exception of Liston and maybe Ali for obvious reasons present time but you get what I'm saying.