Current testnets for the Tezos blockchain are listed here. Read more.
This list of Teztnets is also available in json format.
If you are not sure, pick this one.
Testnets deployed specifically to test new Tezos protocol proposals.
Testnets that restart regularly and track the development of the master branch of Octez repo.
☠️ You probably don’t want this unless you are a core protocol developer.