Upgrade question: Current MacBook Pro 2017 w/Sierra 10.12.6, which OS is it safe to upgrade to?

Backup your data prior to any upgrade.

Upgrading from Sierra, to anything beyond Mojave, know that your applications must be 64 bit as well as drivers.

A 2017 MacBook Pro can upgrade to the current macOS 12 Monterey and to macOS 13 Ventura which is supposed to be released today at some point.