Eliminatus said:
If considering a 100% success chance, just picking a place and make it go boom, Moscow definitely goes first. Hold other in reserve until reactions are analysed. If nothing changes, there goes London.
Also assuming no Allies know of the finite number of them.
Any discussion of targets like Moscow or London assumes that they have an aircraft capable of delivering a WWII era A-bomb, which they didn't. (To be fair, in January 1944, the USAAF didn't either - the B-29 didn't enter service until May 1944.)
The smaller of the two American bombs, Little Boy, weighed 4,400 kg. In theory, the FW-200 Condor could carry up to 5,400 kg of bombs, but that required the use of underwing bomb racks - the internal bay could only handle a max of 1,000 kg. Even if the bomb bay could be reconfigured to handle something the size and weight of of Little Boy (very iffy, given the size of the bomb), the max takeoff weight only allows for roughly 5,700 kgs of crew, weapons, fuel, and payload - and you've just taken up 4,400 kgs of that with the Bomb. You're probably not going to have enough fuel to reach Moscow unless it's one-way mission. London, maybe - much closer target to available airfields.
The best bet for employment would probably be to camouflage it on the ground somewhere in the path of the Red Army's winter offensive, and then use tactical withdrawals to lure as many Soviet troops as possible into the expected blast zone. Even then, you'd probably need a stay-behind SS fanatic to detonate it.