The criteria:
You must be able to carry it.
It cannot be a living thing.
It must benefit a substantial portion of society, not just you. IOW, this is not a get rich trip.
You only get one trip so you can’t break something large into pieces that you can carry.
I’ve thought about this most of the morning and I still can’t decide so I’m interested to hear what others might come up with.
The Holy Grail? Assuming Jesus actually lived, and the events went down as recorded, that’s kind of THE historical artifact of artifacts. Now how I’d verify it? That I don’t know.
If I ended up in 1st century Judaea, and it turns out that there was no historical Jesus, or that things weren’t as advertised, I think I’d probably try to get something of archaeological significance.
Still… a couple hours of video footage might be of more utility than a single item, in that you could see how things were in that time frame.
We’ve already got one.
It’s very nice.
Can I yoink, say, John Wilkes Booth’s gun from just before he would’ve fired it?
A codex selected from the pile that Bishop de Landa was burning
You can try but disarming a crazed gun carries potential consequences.
For believers (no comments on him not being real please), I’d bring back Jesus’s bloody cross, I could bear it long enough to get it into the time machine. Might be interesting to check out the DNA from his fresh non-living blood.
For non-believers, I’m heading to the library in Alexandria (before it burned) and grabbing as many scrolls that I could carry on philosophy, mathematics and astronomy.
Would you be able to distinguish those from say, the records of Ptolemy’s grain purchases?
I’d ask when when I got my library card. 
Imagine the fines I’d be getting when I got back!
Button Gwinett’s signature.
Another vote for the library. With luck, more of Sappho’s work.
I’d bring back some of Jesus’ semen, assuming he existed. (Please don’t ask me how I’d get it.) Half of his DNA was supposedly Mary’s, but the other half?
I think the historical consensus is that Jesus existed. It is more a question if he was actually god that is in dispute.
Forget Jesus. I’m breaking into the Temple of Jerusalem and stealing the Ark of the Covenant, presumably with the original Ten Commandments inside.
Didn’t it take 4 people to carry it?
Fine, I’ll settle for the stone tablets. Maybe Aaron’s rod if I can tuck it under my arm.
A breeding supply of silphium seeds. Except that is disqualified for being alive I guess.
I’d take a camera with me and take some historic (and maybe pre-historic) pictures which I would then bring back with me.