Euclid’s personal copy of his Elements. Alternatively, I wouldn’t mind having Newton’s personal copy of his Principia, except that it’s not lost and is sitting at Trinity College.
Do we have to retrieve? Because I sure as heck would like to go back in time, teach myself how to draw at an earlier age, and help me overcome my crushing self-confidence issues. (Of course, this might just serve to make things worse, or cause some kind of self-fulfilling prophecy, but still.)
The self-consistency filter will frustrate you, sadly. At best you’ll fail miserably; at worst you’ll end up destroying yourself.
That and the DNA from his blood that would be on it would be pretty conclusive, I’d say. (He shot himself with it, remember?)
Have we any tissue samples we know definitely to be Hitler’s?
What makes you certain he shot himself with “his” gun? Nothing I’ve read about Hitler indicates that he, personally, went about armed. Are there, in fact, any records showing H having a firearms license either with or without specific guns listed?
My gut feeling is that 'Dolph (who liked to think of himself as still being a soldierly type) probably just drew a weapon from the armory at the Fuhrerbunker. Lots of military dictator types have had themselves photographed with weapons or even included a holstered pistol in their “uniforms;” but I don’t recall ever seeing a photo of AH with a gun.
All of that aside, the gun Hitler used to kill himself would fetch a princely sum if you could prove provenance.
On the History channel some research indicated that he both poisoned himself and shot himself (seemingly the poison allowed enough time for this to happen). It was the history channel however.
That can be arranged at the same time I’m picking up the gun. 
Also, Hitler’s fingerprints will be on the gun, further IDing it and proving provenance.
Scumpup, I’ve got a number of reputable books on the last days of Hitler and they all agree Hitler was either carrying or had in his desk a Walther PPK handgun for the duration of his stay in Der Fuhrerbunker. So, even if it was a random PPK drawn from the Armoury, for a week at least it was Hitler’s gun. Either way, as you say, it’s worth a fortune.
Incidentally, there is a provenance-proven Walther PPK that belonged to Hitler on display in the West Point Military Museum. It’s Gold-Plated and has “A.H.” engraved on it; it’s purely a presentation piece was it has been proven to have belonged to Hitler.
And now back to your regularly scheduled thread… 
The Russians still have a chunk of his skull in their state archives; and you could probably check mitochondrial DNA against the bodies of Hitler family members who’d share it.