I think Lewis' had more venom. Like if he hit Wlad with his I don't think Wald would still be standing, likewise if Joshua hit that uppercut on Vitali I don't think the fight would have ended shortly afterwards.
The Lennox uppercut looks worse to me, but that might be down to the Rocky style noise when it lands.
I think if The Undertaker faced LL and Joshua in a handicap match and got hit by both at the same time his streak would have ended sooner.
Lewis' was harder. he got better leverage on the shot, his feet were properly planted and he was able to get his whole body into the shot. Joshua's uppercut was brutal but look at his body, he was actually falling backwards slightly when throwing the shot, it didn't have the same leverage that Lewis got behind his. Had Joshua got the kind of leverage Lewis did, Wlad would of gone down.
Joshua's obviously. The Lewis uppercut had to travel longer and higher up to reach its target thus reducing its effect. Wlads head was hunched down when AJ connected with a fully swung uppercut thrown more loaded up and with much greater speed. If you look closely you will see he threw a left hook then transitioned his weight over to his right side to deliver a perfectly formed uppercut with more torque and snap than the Lewis uppercut
Joshuas looked harder to me. If the word hellacious was ever appropriate it's for that punch. Wlad showed IRON chin to stay vertical.
The fans on this board don't even know to look for the basics you just explained. You've just taught a lesson to 80% of the posters
I think LL's uppercut we see Vitali taking on all off the impact. Joshua's to me, looked like it followed through. Well, it did follow through because we see his arm raising into the sky after that blow. I think LL's was more venomous, but Joshua's looked bad ass as well.