Alternate history idea:
"Operation long jump": the Nazis plan to assassinate Stalin, Churchill and FDR. During a conference
What would be the net benefit of this attack for the allied Nations? I mean would their replacements I have benefited their respective countries?
Also, how much of a Target would that put on Otto skornsicsy back? (I know that's not how you spell it, but I'm not going to take the time to look up a Nazis name. It's the werewolf fucker)
Net benefit is IMHO impossible
You can get things which are not bad, and which in the long-run turn out OK and even good, but the impact of the assassination of the main Allied leaders can never be overcome by some simple equation.
Attlee is Deputy Prime Minister, and certainly takes over in the interim. It's up to the King if he wants to ask the Tories to find another Prime Minister from among their number, or if they are willing to serve under Attlee. I imagine that Tories being Tories, once they have a new leader in place they are going to want him to be Prime Minister. There is going to be a period of uncertainty, anger, confusion, and rivalry here.
In the US it is easier to say who is taking over, but it's also a whole barrel of stinking fish because it is going to be Henry Wallace. The political establishment had come to hate him, and the later the assassination the worse things are, because under the US constitution in operation Wallace remained VP until late January 1945, even though his successor on the November 1944 ticket, Truman, would have been VP nominate for 2 months.
The Soviet Union might be even more of a mess. Molotov would be the logical stabilising successor, but whether he could establish his powerbase quickly enough to fend off threats to his own position is another question.