First off, forget Pliney and the monks. Its my assertion that the true "father" of using hops in beer is Saint Hildegard von Bingen. Look her up. Back on topic...
I agree with PBJ. The first "named" hop would have been something like a Golding (or its cousin Cantebury) or something similar from Germany/Czech. But you have to understand that at that time some (or maybe many) believed the cone needed to be fertilized in order to produce a good flavor. That means a lot of hops dropping seeds all over the field. The chance that the hop we call Golding today is identical to whatever was called "Golding" in 1790 is next to zero.
Likewise, in the US, we had the "cluster"...be it English Cluster, Wisconsin Cluster, NY cluster, Pacific Coast cluster...they were just fancy names for a hop that was a cross breed between something brought from Europe and something that was growing wild in the foothills of NY, WI, Massachusetts or wherever.
So the question is, what are you looking for? The oldest grown hop? Not going to find it. The oldest grown hop that still bears its original name? You could probably figure something out but it won't be exactly the same. The oldest variety that came out of the USDA breeding program or some other official program? That you could probably do...actually, I'll see what I can find.