@Amanda1944 There is no "best". It depends on your location, what your guest market is, how far ahead guests usually book in your area, the nature of your listing, etc. There are various cancellation policies because some work best for some hosts and others for other hosts.
Just read all the terms of all the policies and pick the one that you think will work best for you. For instance, flexible, where a guest can cancel up to 24 hrs before check-in doesn't work for most hosts, but if a host lives right near an airport or an interstate hwy, most of their bookings may be last minute, so even if they get a cancellation, they will likely get rebooked on short notice. So flexible might be fine for them.
And you can always change your policy if you find the one you choose isn't working well for you. However, anything you change only pertains to bookings made after the change. Bookings you already have are governed by the terms that were in place when they booked.