Mug level.
Statistic-wise, clay is Zverevs best surface. 3 of his 5 Masters wins were on clay, despite having just 3 clay tournaments per year compared to 6 hc.
Also in terms of career match-winning percentage, clay is Zverevs best surface.
And he now has 2 clay semis at slam level and 3 hc semis, despite there being twice as much hc slam tournaments compared to clay per year.
Yes, but he won Olympic Gold, and two WTF titles on hard court. Those are the biggest titles out of AO and USO on that surface.