Now that Twitter is going downhill, what is the best alternative “tweet” style public social network?

I’ve been seeing a lot of journalists switch to Mastadon. But, the account creation kinda confuses me.

When I go here and try to create an account on the server, it says I cannot, and that I should try creating an account on a different server? Does it matter? Why can I not create an account here? Will I be able to create an account later? Is there a listing of most-trusted-mastadon-servers somewhere?