A lot depends on what you are planning on shooting. I'm primarily a wildlife photographer and if I could only have one lens, it would be my long telephoto. If I were into street photography or landscape, it would be something different. I would recommend getting a native Z mount lens. They are generally sharper, and offer greater compatibility with a newer generation Z camera.
If you either don't particularly need things on the longer end, or are planning on purchasing a telephoto in the near future, you might look at a 24-70 F4 lens. They can be had for under $500 from MPB. This is an S series lens, which are more robustly built, and sealed than Nikon's non S series. This is the lens I purchased with my first Z mount camera, and it has served me well. I was able to use it for macro work with a $40 set of extension tubes. The biggest down side is that it is a lacking on the longer end.
The z 24-120 F4 mentioned above would be a good choice as well, although I have no personal experience with it, and it does run a good bit more used. The likely reason for that is that the z 24-70 F4 was offered as a kit lens deal on a lot of Z bodies, whereas the z 24-120 was not, to my knowledge. Right now the z 24-120 F4 is on sale for $1000, which is very close to the current used prices.