Do "best in the world" and "best ever" mean the same thing? For example, "best father in the world" sounds fine to me, but "best father ever" sounds a bit odd to me. It sounds as if a person has had more than one fathers, and the current one is the best. I'm not a native speaker of English, so I might be wrong.
They are both common figures of speech and mean the same thing.
The best father in the world
does not at all mean "best of my fathers living on earth." Rather it literally means, through ellipsis, "the best father out of all fathers alive on earth." Similarly
The best father ever
does not at all mean "the best of my fathers, living or dead." Rather it literally means, through ellipsis, "the best father out of all fathers back to the beginning of life on earth."
Of course, neither is intended to be taken as assertions of literal truth. They both are hyperbole and are intended to be interpreted as "excellent."