It's pretty notable that Zverev has 0 Wimbledon quarter-final appearances, in contrast to Stich reaching the semi-finals or better at all 4 majors. That's despite the fact that Stich played in far more polarised conditions, not just in terms of surfaces but also in terms of having to adapt to facing opponents with a far greater variety of different playing styles, and also when there were 16 seeds at majors instead of 32. The switch to 32 seeds from 2001 had a pretty big impact on the sport, not least in terms of reducing the potential early round banana skins for the top players.
In terms of talent, it's a no-brainer to me that Stich comes out on top there.
Zverev is what I'd call an 'academy style player'. He does the fundamentals very well, including moving well for his height (though Stich himself was an excellent athlete). But he lacks the ability to 'improvise' and be spontaenous, and of course his net game is not exactly strong for top level professional standards.
In 1991 when he reached the semis at RG and then won Wimbledon, the likes of Newcombe and Courier (who he played in both tournaments), were very complimentary about Stich's game, his variety and ability to adjust his playing patterns so much.
The fact that he enjoyed such a successful career, despite committing fully to tennis so late, was also notable. When he was 16 years old a few months out from his 17th birthday, and watched Becker's incredible 1985 Wimbledon triumph, tennis was still only his 2nd main sporting focus behind football. At that same age, most future pros have basically been full-time ball-machines for several years at least, and eat, drink and sleep tennis.
