Is it me or does German have more different words for this than most languages? Etage? Geschoss? Just Stock, which I guess is short for “Stockwerk”? In English we only have “floor” or “story” and I’m not aware of any other way to say it.
Geschoss requires a prefix to sound natural (Obergeschoss or Erdgeschoss). It would sound slightly off to say Ich wohne im fünften Geschoss.
Stock is (strictly speaking) informal language and should always be used in combination with a number.
Etage should be used without a number and rather when referring to the floor you’re currently at. At least it is rather uncommon to refer to the fifth floor as fünfte Etage or to refer to a floor you’re not currently situated at using this word.
Stockwerk is the only one that is universally usable.