Im with Mo on this. My "one item" would be a duffle bag filled with whatever I could fit. We also have the luxury of knowing what all these items are worth now and how scarce some of them are. But if I had to pick one item, it would be a complete black pre-war SS uniform with parade black DD helmet, the best of both worlds. They just look bad ass. Close runner up would be a Knights Cross with oak leaves, swords and diamonds with the award document (bonus would be the recipients complete uniform).
Ask me again tomorrow, it will probably change.