autocoronarse

To crown oneself, literally (a historical or ceremonial act, echoing Napoleon crowning himself emperor) or, much more often today, figuratively and critically — a person or group declaring themselves the best at something without real authority to back it up.

• El emperador decidió autocoronarse en vez de dejar que lo coronara el Papa. — The emperor decided to crown himself instead of letting the Pope crown him.
• El equipo se autocoronó campeón antes de que terminara la temporada. — The team crowned itself champion before the season had even ended.
• Se autocoronó como la máxima autoridad del tema sin que nadie se lo pidiera. — He crowned himself the ultimate authority on the subject, unasked.

Missing the critical, slightly mocking tone the figurative use almost always carries — outside the historical/ceremonial sense, autocoronarse is rarely neutral.