Awhile back I started working on (but never finished) a project to find accurate P99 ZEMs. As part of it, I did a lot of research to try and find the zone with the best level spread.
It looks like South Qeynos might have the best spread (or Freeport if you count all it's zones as one). If you're curious you can see all my research here:
https://wiki.project1999.com/Zone_Zero
One bit that might interest you is that at the end I put some code you can use to see the levels in a zone. If I wasn't on my phone, I could use it right now on Rathe Mountains ... but since I am, I'll leave it up to you.
Regardless, good luck on the challenge!