I know many would choose Mercer for sure, but he lost to worse opponents, this would be a great fight...
Parker also lost to Whyte, should've lost to Chisora 1st time, and won a controversial decision over Ruiz. I don't think any of those fighters are necessarily better than Mercer. But saying that Parker does have a solid chin and his movement and underrated boxing skills would be frustrating for Mercer. Both fighters at their peak ? Probably a close decision either way I might give the slightest edge to Parker.
I don't see a path to victory for Parker. Mercer's last three losses were after the age of 40 and some long layoffs caused by injuries. He was robbed vs Lennox Lewis.
Parker is a big favorite to win this one for me by decision. More mobile. Better boxer. Height and reach. Decent chin