Take Berlin Hostage.
This wouldn't win the war, but it's a more interesting answer than "nowhere", and in line with Hitler's belief that he was fighting the Götterdämmerung.
1 April 1945: Germany Has Already Lost.
The German situation on April 1st, 1945 was unsalvagable. Their army was in ruins. Their capacity to make up for losses done. All the men and material necessary to invade Berlin and end the war were already in place. And they were being attacked on two sides by multiple armies. There is no one place to strike.
By April 1945 the war in Germany was over. The Germans faced overwhelming odds on all fronts. The Soviets had 2.5 million soldiers ready to assault Berlin defended by only 750,000 under-equipped and worn out Germans. They were already over the Oder River and preparing for the final push.
In the West Army Group B was surrounded in the Ruhr Pocket and the Germans had lost their industrial heartland. They had already crossed the Rhine and were driving east and south. The Western Allies voluntarily stopped at the Elbe River (the Soviets were promised Berlin) and turned south to prevent a possible Axis retreat to the Alps.
Attack the West?
Maybe a port of supply like New York? The effect won't be felt for weeks. Maybe a capital like London or Washington DC? The political decisions had already been made, and the Allied high command was too flexible for a knockout blow like that. Maybe a port in Europe like Amsterdam or Rotterdam? The Western Allies now had many ports to choose from, and a good supplies built in.
More to the point, attacking the Western Allies won't stop the Soviets, so any target in the West is right out. You have to strike the Soviets. But where?
Moscow? Delay the Soviets, Surrender to the Allies?
Striking Moscow seems the most logical. Soviet government was very centralized, and Stalin had a personal hand in many decisions. With the Supreme Soviet decapitated, the Soviet military and government would be in chaos. There's a slim chance this could lead to an anti-Stalinist uprising, or a civil war over control of the government. But more likely the hatred for the Germans and final victory being so close would at least delay this fight until after Berlin is taken.
Moscow was also a major rail hub. Its loss would throw the already shaky Soviet logistics into further disarray. Even so, they had the men and material already poised to strike Berlin. Such a disruption would be too far back in the supply line to affect the war in the coming weeks.
Still, if you have one throw of the dice this is it. If the Soviets proved unable to take Berlin the Western Allies would happily do it for them. At least the generals would be happy for the glory of the prize, the tends of thousands of soldiers who would die in the assault would not. For the Germans, they would find the Western Allies far more forgiving than the Soviets.
How To Get The Bomb There? Or Anywhere?
One problem: the Germans had no way to get a bomb to Moscow. Or New York. Or anywhere.
A nuclear bomb in 1945 was an extremely heavy, bulky item. Fat Man weighed 10,000 lbs. It required the largest, most powerful, and most advanced bomber in the world, the B-29, to carry and drop it. Even then it had to be modified to carry the bomb. The Germans had nothing like the B-29.
Even if they had a bomber, how could they make it thousands of miles through Allied dominated airspace?
Put it on a V-2 rocket? Again, too large, too heavy. The V-2 had a warhead of just 1,000 lbs.
Only One Bomb.
Even if Hitler gave the plans for a working atomic bomb to, say, Japan, Japan would not be able to produce a bomb of their own. Neither would Germany. They'd have the one bomb, that's it. The problem is refining the nuclear material is extremely slow, expensive, complicated and energy intensive. And in 1945, nuclear bombs were extremely inefficient with this precious material.
One of the aspects which is underappreciated about the Manhattan Project, and why it was so expensive and took so long, was the production of enriched uranium or plutonium in the amounts needed for a nuclear bomb is extremely difficult. So difficult and so important that the Manhattan Project set up TWO enormous facilities to produce it using two different methods.
Neither Germany nor Japan had anything like the facilities to do this, nor the safe space for such a large and fragile facility. If we assume they can scrape together enough material for one bomb, we cannot assume they can make another.
With nowhere decisive to drop the bomb, and no way to get it there, Hitler would be left with one option: threaten to blow up Berlin. While the Soviets would probably be happy to see Berlin and everyone in it wiped off the map, they would not be so happy to see a million of their soldiers vaporized in the process.
This would lead to a very awkward situation. If Hitler could convince the Allies he had a working bomb, he could retain control of Berlin while the Allies finished off the rest of his empire. The Allies would set up a blockade and Berlin would begin to starve. This stalemate would not last long.
Eventually, someone sensible on his staff would assassinate Hitler. Someone not eager to see Berlin destroyed by atomic fire or starvation. Someone like Albert Speer, Minister of Armaments and War Production, who famously tried to salvage as much of Germany's infrastructure as possible in the closing months of the war.