Mohammed bin Salman (MBS) is the crown prince and prime minister of Saudi Arabia. Although some press outlets referred to MBS as "de facto leader" as early as 2015, the term appears to have only become consensus in 2017, with the label used for example by The New York Times and The Economist.

This question relies on a potentially ambiguous term (de facto leader) and therefore resolves according to credible sources and, where necessary, my personal judgement.