I prefer whatever the Czechs prefer. Since there was no referendum, it's hard to say, but seems like they prefer Czech Republic to me.
Czechs themselves use the word "Česko", which I guess, most directly would be rendered in English as "Czech". Nobody is proposing to rename the country in its own language. And I doubt most Czechs speak English sufficiently fluently to have a position on how the country should be named in the tongue we are speaking here.