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.
Possible clarification from creator (AI generated): If MBS becomes King, he will still be considered de facto leader for the purposes of this question.
Possible clarification from creator (AI generated):
De facto leader status will be determined based on evidence of intangible power held by MBS and mainstream press reporting, not solely based on official titles
If MBS becomes King, he will still be considered de facto leader for the purposes of this question
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1.00@cash I think it is a misnomer to call him the "de facto" leader because he is just doing the job of the Prime Minister, but nobody would credibly call any previous prime minister the leader, nor would anyone mistake the crown prince who wasn't the prime minister as the leader. Having both titles is not a leader, he is more like the spokesperson for his father. But in reality his father is still very much in ABSOLUTE control and could take it away everything MBS has at any moment without any reason. If his father was incapacitated and unable to weigh in on issues then he should be considered de facto, but not now. When foreign dignitaries come to Riyadh there is a clear difference in how they are treated and nobody would mistake MBS for the King.
@datachef MBS is widely reported and referred to as "de facto leader" of Saudi Arabia in mainstream press and other sources like Wikipedia. As I understand it, MBS has substantially more power than the King and I am skeptical that his father is able to effectively exercise his technical authority to remove MBS. His father seems ill, having been hospitalized recently for lung inflammation (see this CNN article, which not incidentally refers to MBS as "de facto leader").
You say that nobody would credibly mistake the crown prince or prime minister for the leader, but this is exactly how Wikipedia, CNN, The Economist, and others refer to MBS. It seems substantively accurate to me, but if you still disagree, that's all good, you are welcome to not bet on this question. Or you can bet on this question with the understanding that "de facto" resolves to a judgement call about the intangible power held by MBS, not official titles alone, and that my current best judgement is that the evidence warrants calling MBS de facto leader.