World's Best English Teacher ... you absolutely do need the apostrophe Also, your question should be, "Does the following sound good/ok"

I don't think that you need the apostrophe. As it is a replacement, by using it you are saying "the world is greatest teacher" without the apostrophe it would mean belonging to, so " the worlds greatest teacher" Lovely thing to do for your teacher :-)

You can just say, "World's Best English Teacher!!!!!"

World's Best Teacher of English Certificate We're just preparing a present for our English Teacher and I want to clarify some details. :)