If a sentence omits a noun in a structure like this, it means that the writer is avoiding repeating the SAME noun. The Eiffel Tower is the most impressive in Paris.=The Eiffel tower is the most impressive TOWER in Paris. (Maybe the Arc de Triomphe might be the most impressive MONUMENT) Korean beef is the safest in the world=Korean beef is the safest BEEF in the world (maybe Italian celery is a safer FOOD). In general, we try to avoid repeating ourselves whenever possible, so we will often omit repeated words in parallel structures like this. Personally, I would definitely say "Korean beef is the safest in the world" instead of "Korean beef is the safest beef in the world". However, your first example felt a bit strange to me. When a word is part of a name, it is usually OK to repeat it outside of a name. So, to me, "THe Eiffel Tower is the most impressive tower in Paris" sounds better than "The Eiffel Tower is the most impressive in Paris." I would only use the version without "tower" if another nearby sentence or clause already used that word. For instance: "Many cities in the world have become associated with a single impressive tower. The Eiffel Tower is the most impressive in Paris, and the Sears Tower is the most impressive in Chicago."