The best ZX Spectrum emulator that I found for macs is Fuse for macOS.
The latest version of Fuse for macOS is 1.5.6 which was released in August 2019. Even though this is a little old, it seems to work fine in 64-bit only macOS Catalina.
All of the main ZX spectrum versions are available, from 16K, 48K, 128K, +2, +3, together with some more unusual versions. (Pentagon 1024K anyone?)
Image Credits: By Bill Bertram – Own work, CC BY-SA 2.5