Gassiev. No doubt in my opinion. Haye was quicker and more aggressive, but at CW, he was hurt multiple times and he could always get a bit wild. Gassiev's chin was tested massively vs Dorticos, so we know he get take one to land one. I think he catches Haye with that left hook and knocks him out.
Need to see more of Gassiev. If he gets close he destroys Haye, but Haye had serious power in that right hand counter and it was much straighter and quicker than what Dorticos could throw. Haye wasn't great by any means, but that one punch of his was
Gassiev has had some awesome fights. Lebedev - coming off his best performance too, IMO - and Dorticos are both pretty fun. His KO of Shimmel is horrifying. Krzysztof Wlodarczyk is probably his best performance, from memory. Haye vs Gassiev would be an awesome fight. Haye was a very entertaining fighter at CW, IMO.
Id lean toward gassiev. Seems like he's sturdy enough to survive the occasional hayemaker, and as said, up close, he rips haye
Gassiev in a P4P sense is Usyk's best win. I mean, AJ would beat Gassiev in absolute terms, but I definitely don't think AJ is a better fighter. I don't think any joe blow would beat Gassiev at cruiser. Only history's elite cruisers. I mean, obviously Holy would eat Gassiev for breakast.... but Haye ain't Holy.