Uhh there's not really a "best offset" lmao. offset really depends on your hardware and to an extent your own preference.
If you find that you're consistently hitting late or early to an extreme degree, change your offset to help with that. Otherwise, you'll want to use your skin's skin.ini to change the hitposition instead.
Typically there's not much reason to change offset unless it's local. Local only applies to that specific beatmap and can help with maps that weren't correctly timed.