OK, tough question, but I will go for SRK
But before users here attack me, let me give the reason, First of all, SRK just went inside the character of both Aryan & Gaurav, and two characters, so different from each other in everything, plus how srk manages both characters, his transformation from Gaurav to Aryan and Aryan to Gaurav don't seems force, from nowhere it feels that both characters are played by one man, and if you want example, take the jail sequence and just watch it again and again, when Aryan and Gaurav meets one another and see how srk not only handles himself but also the characters and then his voice as Gaurav and changing them into Aryan's voice in few scenes, Gaurav was one of the most challenging role of SRK's career and he pulled it off perfectly, and what's great about SRK is that even though Gaurav was the challenging one, he played Aryan Khanna too with perfection, never it seemed that SRK ignored his Aryan Khanna for Gaurav, such an act it was..............
GAURAV is ARYAN, ARYAN is GAURAV............
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Now coming to Leo's performance, we all know that it won Oscar, but compared to SRK in Fan, there is one major flaw in his performance that makes srk the winner, and that is.............. I haven't watched Revenant, so SRK is the Winner lol :D :D :P :P