Jon Jones. 100% of the time, every day of the week plus Sundays. Even the sub top 20 170lb UFC level fighter would beat the Rock. Most of the top 145–155lb fighters at UFC level would beat the Rock. Below 145 it gets hard to imagine that little weight and heighe being able to deliver enough force to KO a guy like Rock, but they'd be good enough to not get beat up by him…jab, kick and flee. Big is good if you're trained, but being trained/naturally gifted equivilant to a UFC fighter is rare, first of all and secondly Rock isn't a fighter…so he's just big. Strong, athletic , good actor and big…but not a killer.