Wladimir Klitschko of the Fury fight vs. Andy Ruiz Jr. of the Joshua fight.

Joshua basically copied how Wlad would have fought him in the second fight. Joshua's punch resistance is worse than Wlad btw.

 

Wlad would stop Ruiz without trouble. Ruiz would have the smallest stature and reach of any opponent he'd fought and the least power.
People suddenly think because Ruiz got up from Joshua's hook that his chin is invincible and so he'd last with the biggest puncher at HW. He's barely been tested by big punchers. Wlad would also land and also put Ruiz on his ass. But he'd keep doing it until the fight was waved off.

If smaller guys like Haye and Povetkin (still taller more powerful and explosive than Ruiz) can't land clean enough to do anything, Ruiz won't land a glove. Haye and Povetkin are getting airbourne trying to land, Ruiz can't get an inch off the ground.

Wlad would swell Ruiz's fat head so badly, the blubber would start erupting from broken skin. They'd have to get a special cut man to deal with the oozing fat, maybe a liposuction specialist. By the end of the fight the ring would look like the inside of Ruiz's arteries, and globules of lard would drip from the ropes.

 

Assuming you mean Ruiz Jr from fight 1

Wlad would box him to death and win a wide points victory, Klitschko fought cautiously at the end of a long jab and straight right. He’d keep that up for the full 12 rounds and wouldn’t risk getting caught. It would look very similar to the Ruiz Jr V Joshua 2 fight

Joshua lost the 1st fight because he threw caution to the wind after knocking Ruiz down and mistakenly thinking he was badly hurt.