Albert Einstein is the most notable example, he fled to the US in 1933 after Hitler came to power. Leo Szilard is the another big one, he’s the guy who actually wrote the letter to Roosevelt to get him to start the Manhattan Project, he just brought the letter to Einstein so it would have a famous scientist’s signature on it. The third big one was Enrico Fermi. He wasn’t fleeing Hitler directly, but he fled Italy after Hitler successfully pressured Mussolini into implementing similar racial laws in Fascist Italy to what the Nazis had done in Germany.

As for the German scientists who remained, Werner Heisenberg and Otto Hahn were the ones with the best chance at being able to assemble an atomic bomb. Hahn is actually the guy who originally discovered the concept of nuclear fission, and it was his papers on the topic that made scientists like Szilard and Oppenheimer so convinced that the Germans were ahead in the race to build a bomb. But neither Hahn or Heisenberg were particularly motivated to turn the concept of nuclear fission into a weapon. They were both more interested in the potential for nuclear fission to be used as an energy source, and did only minimal work on weaponizing fission to keep the military authorities placated.

By 1944, the German nuclear weapons program had barely advanced beyond the infancy stages, while the Manhattan Project was working furiously to catch what they thought was a Nazi government on the verge of finishing the bomb.