3D Monster Maze was *the* game back in the day and I understand that in more recent years young retro gamers have come to appreciate it too. Some play it with huge screens and describe the moment the T-rex appears on screen as "pants wetting".
Other games I'd specifically recommend for the ZX81 are the more recent "Mayhem"
https://www.homebrewlegends.com/zx81-mayhem/
I still get kicks out of "Perilous Swamp" from the "Fantasy Games" cassette, and Catacombs another JK Greye title.
Artic's Galaxians was great
Mazogs