How do I choose the best iPad for optimal ZBrush performance?

their app site says

Compatibility

iPad

Requires iPadOS 17.0 or later and a device with the A12 Bionic chip or later.

ZBrush on the App Store

as with most things 3d then the bigger cpu(gpu) and more system memory the better so

if it was me then I would look at an iPad pro

https://www.maxon.net/en/requirements/zbrush-requirements

Minimum Requirements:

• iPadOS 17 or later
• Generation 1, 2, and Pro Pencil
• USB Pencil (no pressure sensitivity)
• A12 Bionic or later
• iPad 10th Gen+, iPad Air 4th Gen+, 12.9” iPad Pro 4th Gen+, 11” iPad Pro 2nd Gen+

Recommended Requirements:

• M-SERIES iPads or later