It was extremely cheap to buy slaves and maintain them. It was also beneficial to slave owners because they could sell slaves for more money than they got them for. On top of this slave owners automatically owned any children their slaves had while under their ownership.
Tokyo is the answer you are looking for. The second best answer is Lisbon, but Spain (from Hitler's point of view) did not need to be invaded. It was a fascist country and it likely had nothing he wanted. When you think of Hitler's needs, think oil oil oil and more oil. That's basically why he invaded Russia which was the most horrific part of WWII next to the Holocaust.
