You could say Η ελευθερία του λόγου (so both are correct Greek). It's also accepted as an answer.
I think that the terms are Freedom of Speech without article and Ελευθερία του λόγου with του. Babiniótis and wikipedia supports me
Εντάξει, if you say so. Some people say that OK comes from εντάξει. They are probably joking. Already Plato used it however, as εν τάξει
Do we say "Endaksei" or Edaksei", I've heard both of them but I couldn't know which pronunciation is right!
It is not a matter of grammar, but how accurate the phrase is within a socio-political context. "Ελευθερία του λόγου" is the correct form, as it is a solid legal term that refers to a fundamental human right. That being said, native speakers use both terms in everyday life.
Terms like "ελευθερία κίνησης" and "ελευθερία της κίνησης" (freedom of movement) are interchangeable.