zlib1.dll
This file is not a system component: the program that bundles it, not a system package. Restore it from the source that installed it rather than downloading a replacement.
Do not download this DLL on its own
Searching for a missing DLL by name leads to sites that hand you that single file. It is one of the oldest and most effective malware delivery methods there is, and a system DLL from an unknown source is indistinguishable from a deliberately modified one. There is never a legitimate reason to obtain a runtime component that way.
What it is
The zlib compression library, one of the most widely used pieces of software in existence. It is open source and applications bundle their own build of it; Windows does not provide one.
Where you see it: Graphics tools, emulators, git ports and anything that reads compressed data.
Reinstall the program that reported the error, because its copy is the one that matches. There is a second trap here specific to zlib: several programs ship it under different names (zlib.dll, zlibwapi.dll, libz.dll) and they are not interchangeable, so renaming one to the other produces a program that starts and then crashes.
Why there is nothing to download here
There is no verified download we can offer for this one, because installing a file is not the fix. The steps above are what actually resolves it. That distinction matters: the sites ranking for this file name are selling a download that would not have helped even if it were safe.