This is the 14743rd most frequent German word.
oberste
"Uppermost" or "highest."
In this context, 'oberste' describes 'Instanz' (authority), indicating the highest level of decision-making body.
Die Entscheidung liegt bei der obersten Instanz.
The decision is up to the highest authority.
Here, 'oberste' is used as a superlative adjective to qualify 'Regel' (rule), indicating the most important or topmost rule.
Die oberste Regel ist immer Ehrlichkeit.
The foremost rule is always honesty.
'Oberste' is used as an adjective describing 'Etage' (floor), referring specifically to the highest floor level.