OTL Molotov was the one who dared to do the most toward standing in for Stalin while he was in his drunken funk. Lavrenti Beria, as head of the police, is in the best position tactically though not so much politically to seize supreme power.
I'd say that in the short run, it depends on whether Beria attempts to do so immediately. He'd probably fear that no matter how serviceable he is to either some other individual or a collective leadership, they will turn on him if he doesn't get them first because everyone feared and hated him. Then again, OTL he was in much the same position when Stalin did die in 1954, and yet he thought he could form a coalition with other Kremlin insiders* and was taken by surprise by the triumvirate of Molotov, Malenkov, and Khrushchev.
I can't name anyone of the latter two's status as of '54 to play an analogous role in '41; the Party leadership was pretty well beaten down at that time. Obviously Zhukov was in no position to take leadership then; he had neither the status nor the free time to play politics in Moscow, what with trying to fight off a Nazi invasion and all.
Nor would either a dictatorship of one (probably Molotov) nor a ruling committee dare to turn on Beria just yet, again with the big war on.
So I'm guessing a Molotov/Beria duo, Molotov taking the lead in public but everyone knowing Beria was lurking there in the background.
I wonder who Molotov would send on diplomatic missions to the West, as Stalin sent him OTL. Certainly not Beria! Those two would stay in Moscow watching each other very carefully!
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*Beria even thought he could share with other insiders his notion that at that point--1954, when Stalin died OTL--that the Soviet leadership should simply dispense with the pretext of Communist rule, abolish the Party, and simply govern through the State, as ministers of the Soviet Union. I'm sure they executed him as much for his heresy of his for the rattlesnake-like existential threat he posed for all of them!
In 1941 he would say no such thing and probably not think it.