I haven't seen Greb can't comment. Never seen Charles at Middleweight. The only one I'm sure Golovkin would beat is Pavlik. Leonard I'm not sure about Middleweight wasn't his best weight but I could maybe see him edging a decision over Golovkin being the flashier fighter. Nunn didnt have a long prime but at his best between 1988-1990 he would be close to even money against Golovkin. The other names on the list I wouldn't make Golovkin a favourite over any of them.
Same take here. Regardless of how anyone feels about the results, Canelo showed enough against GGG that high IQ and ring smarts and a solid beard level the playing field a goodly amount on GGGs power and ability to cut off the ring. I actually think the worst ones on the list for GGG would be Nunn, Hopkins and Toney - bad style matchups for Golovkin. Obviously the pick of the bunch would be Hagler vs GGG - a war for the ages that!
The only one I'm certain he beats is Pavlik The best case is Leonard who was older and above his best class and Nunn though I have not seen much of Nunn I think Golovkin is a great fighter but I don't think he is as great as the legends listed above.
Good call among the posters here, of the 10 fighters listed Pavlik is the only one I see he beats for sure. Not saying he couldn't get another win here or there , but I don't think he matches up well with them at all