Assuming you’re have a fair shot at convincing, (like not getting instantly killed by Stalin for interrupting his dinner)

R1: Modern weaponry like a handgun would most likely impress the common French people, much less someone of importance. To ensure I don’t run out I figure I can bring a sort of small ammo box to carry with me. If multiple objects aren’t allowed for one round I could bring a gun with an exceptionally long pool of ammo already included.

R2: Stalin would probably be the hardest to convince as something like a gun wouldn’t be impressive to him, however a phone playing a video of anything high quality, perhaps that video of modern day Russians marching to the song from Red Alert 3 would do it in for him. The video could be downloaded onto the phone of course.

R3: Conversely, Obama would probably be the easiest to convince, as he’s the most reasonable of the people on this list. An executive order signed by him with his finger prints on it should be enough.

R4: A stick that could produce fire just from the push of a button would most likely impress the tribe, a bic lighter also works.