It might have been the $500 ebay example Ed mentions above, but as he didn't save the pic, it looks like we'll never get to see it!
Dream on. You left out one that I consider important. Perseus must hold a weapon that shows the curved hook appropriate to the Harpe he was given for the task. The best coin will show that weapon including the hook. Zumbly showed the best one on CT. https://en.wikipedia.org/wiki/Harpe
Fantastic coins everyone, here is my mutant: Pontos - Amisos. Mithradates VI. Circa 85-65 BC. 28 mm / 19.07 g
I found the example I was remembering. Sold for EUR 417.60 (US $483.27) on eBay, April 22nd. I think it has everything @Valentinian and @dougsmit asked for. It is from Sinope which is a mint I don't own for this type so that would have been a plus. The Perseus isn't particularly artistic though.
Even so, it is far from flawless. The headless body is small, the facing heads have few details, and Perseus does not look strong. The sword is outstanding, as is the obverse. I'm holding out for a better one!
What we have here is the coin version of an old theological question. Is it 'Greater than which nothing can exist' or 'Greater than which nothing can be conceived to exist'? I suspect that the finest example of this coin will not be perfect in every one of Valentinian's criteria (and my one). Certainly Svarog's coin is the best Harpe I have seen (Grade A all the way!) but ranks B or C in all the others. I suspect a coin with straight B levels on them all might well be the best overall coin that actually exists. It is very hard for a coin to be bold on all four compass directions. I wrote a page on the impossibility of grading the crocodile coins of Augustus and Agrippa. The Medusa type would serve just as well for the purpose. Collectors too good for less than absolutely perfect coins should not collect ancients and must not collect the headless Medusa. There is a trick. Some cities used larger flans than others. Amisos is the most common but tends to have smaller flans. Finding a perfect, full flan set from all cities is not going to happen.
Hoping it's not too late to join the "Headless Medusa Club." I never thought I'd get one of these (too expensive), but I found a cheap(ish) one on eBay. It has obvious problems, but I liked the full-body Medusa and the hook on the harpa. Despite the obverse environmental problems, I thought the obverse Athena was quite nice in terms of style. Anyway, I found this post very helpful in evaluating the appearance of these. Thank you all for sharing so many interesting & attractive examples. Pontos, Amisos Æ 28 Mithradates VI the Great (c. 85-65 B.C.) Helmeted head of Athena right / AMI-[ΣOY] across fields, Perseus standing facing, holding harpa and head of Medusa; Medusa's body at his feet, M(T)E and ΓA monograms left and right. Malloy 16e; RecGen 17c. (19.14 grams / 28 mm)