Prior to installing newer firmware update make sure to first install the latest version of jailbreak hotfix (that has changes & fixes for >= 5.15) from the
snapshot thread and do 2 extra reboots / restarts after that. After that you're safe to update to desired firmware version (and then again re-do the hotfix installation + reboots).
As for what firmware version is "the best" that is highly subjective thing. I could only point out couple of important firmware versions that introduced important UI and/or functionality changes.
Like the fact that many many longtime Kindle owners hated the changes regarding collections that were introduced with firmware 5.14.3 IIRC.
Anyhow, as long as you don't lose the jailbreak in worse case you should always have an option to do a "downgrade" firmware to a particular version you liked the best and/or considered the last good firmware.