The first “Flowing Hair Dollar” coin minted by the US in 1794, in mint condition.
It’s a toss up between the Ark of the Covenant and the Amber Room. Maybe the Ark of the Covenant inside the Amber Room?
But remember, you can’t actually TOUCH the Ark, or you die. (Unless you’re a Levite.)
Not true, I saw in that documentary called “Raiders of the Lost Ark” that you can touch it, but not open it and certainly not look inside.
Wot, no “lost in the wash” joke? 
Aaron’s rod?
The Spear of Longinus?
Hell, I’d love to uncover one of the weapons stashes the Soviets allegedly stored in several locations in the United States. One of them was allegedly in northern MN. Of course, I’d love more to not be killed by the booby traps.
The bill from the Last Supper.
James Dean’s “Li’l Bastard” Porsche
Unopened case of Topps 1952 baseball cards, possibly containing Mickey Mantle’s rookie card.
The lost photographic plates from the Shackleton Expedition.
Hirohito’s cup - this is the one he took a drink from prior to formally surrendering, it was the last item that he touched as a divinity, after the surrender he became a mere mortal - that cup is missing
The sword of Attila the Hun.
The casket (and mummy) of Alexander the Great.
A minty fresh 1967 Chevrolet Chevelle SS with a 396 engine. Fire engine red would be a bonus.
The Griffin. No great treasure but I’m totally nuts about the history of the French in North America.
A Carlos Hathcock "In Country " Sniper Rifle .
Good one. I don’t know if it really counts as a historical item but I want one of the Strawberry Leaf variety Wreath Cents. This particular variety has fascinated me for some reason. Only 4 known to exist, all severely worn.
Wright brothers first plane they flew.
Lincoln’s “lost speech”:
Legend says it was so moving, reporters in attendance laid down their pencils and forgot to take notes. People who heard the speech have asserted it was Lincoln’s greatest, but were unable to give details. Some think it was deliberately suppressed out of fear it would sunder the Union and lead to civil war.
It catapulted Lincoln into political prominence, so in a sense it did. I’d much rather have that speech – the best of his life, from an acknowledged master of American English – than a coin or a gem.
Since the Ark and the tablets are taken as it where, I’ll make do with my family’s own collection of military or quasimilitary commendations. One of my great-grandparents on the maternal side was a Guardia Civil, it’s a police body with military structure and which follows military regs; the paternal side spent a milennium going to war any time king and country called for it.
Those on my paternal side went to cousins which are distant both in the family tree and in space; those on the maternal side were never collected to begin with.
I’d be interested in seeing Ismail al-Jazari’s robot band.