Best way to run Microsoft Access on iMac?

Depends on what kind of use you will be making of it. Either way is fine ion you know the differences.

Using Parallels will share resources with Windows so may be a bit slower than Bootcamp.

Bootcamp will let you boot into a full version of Windows, that can run any Windows applications, but you won't have access to the Mac side until you reboot and boot into macOS. Bootcamp makes them entirely separate.

Parallels on the other hand will let you run Windows in a window on your macOS desktop and you can just use macOs and Windows at the same time, but they will share resources, so can be a bit slower.

If you don't need to run massive Access databases that require the full use of all the Mac's resources, Parallels will work fine.

Otherwise go for Bootcamp.

Also note, that Windows on neither Parallels nor Bootcamp is supported in the 2020 M1 Macs and 2021 iMac (with Apple processors) at this time.

If you are not necessarily married to MS Access, consider OpenOffice (http://www.openoffice.org/product/index.html) or LibreOffice (https://www.libreoffice.org) alternatives.

There are other database engines - SQLite3, MySQL, Postgres.