Wherever Hitler is.
Hitler will never surrender. The Nazis are loyal to Hitler and will fight on as long as he does (as in OTL). The Army is in a similar state of mind. Though a significant faction tried to get rid of Hitler to end the war, they failed and the rest of the Army fought to the end. But it's all held together by Hitler's will and personal command. (Much of the Army considered itself bound by the their personal oath of loyalty. You may consider that foolish, but to them, the whole point of being a soldier was to perform one's sworn duty, whether or not one liked the commander. Kill Hitler, that goes away.)
Nuking Hitler decapitates the Nazi regime, and demonstrates an irresistable weapon. That's all that's needed.
Nuking central Berlin won't prevent a German government from surrendering; the fall of Berlin in 1945 did not prevent a new government forming in Flensburg, which surrendered a week later.
Also, Hitler may not be in Berlin. If he's at Wolfschanze or Berchtesgaden, nuke that.
The B-29 or equivalent is not required to deliver the Bomb. The Bomb could be parachuted down to a soft landing, and detonated after 10 minutes by a timer. (No, the Germans are not going to be able to rush up and disarm the Bomb in ten minutes. If that is a real worry, have a bunch of small fragmentation bombs on the outside of the case, with a timer that detonates one every 30 seconds. That will keep any German UXB crew at a distance.)
It's possible that parachute technology of the time could not land a Bomb-size load with less shock than a Bomb could tolerate, and that an airburst is required.
OK, do this. Build the Bomb into a Lancaster. The Bomb plane flies under tow from a second plane which pilots the Bomb plane by wired remote control. (The two planes take off together; the Bomb plane take-off crew bails out after reaching cruising altitude.) 25 km from the target, the controller arms the Bomb, locks in the Bomb plane autopilot, and starts a timer. The tow/control cable is dropped, and the guide plane turns back at top speed. The Bomb plane continues for five minutes - then demolition charges shatter the fuselage, letting the Bomb fall free to the target for an air burst.
Some other targets have been suggested, some quite fatuous.
The Ploiesti oil fields cover hundreds of square km; a nuke would damage only a small part of it. In any case, the entire area was overrun by the Soviet Army at the end of August - but German resistance continued for eight months.
U-boats were built at yards in several different cities: Emden, Bremen, Hamburg, Flensburg, Lubeck, Kiel, Rostock, Stettin, and Danzig; nuking one site would have little effect. And by August 1944, U-boat operations were a net loss for Germany: the number of U-boats sunk was about equal to the number of merchant ships sunk.