Thank you for the opportunity to help you with your question!
Well first he annexed Austria. then annexed parts of Tsjecho-slovakia(Sudetenland). Then (after promising at the conference of Munich that Sudetenland was his final territorial ambition) he demanded the Danzig Corridor from Poland(Danzig itself was a free City under the league of nations). after refusal he invaded.
So it would be Poland.
Please let me know if you need any clarification. I'm always happy to answer your questions.