I'm going to change my answer, now that I've thought about it. U.S. coins do have a sustained demand beyond other numismatic markets, and that's unlikely to change at any point in the near future. So that said, I'm going to simply say that the most overpriced U.S. coins are Morgan Dollars, Peace Dollars, Silver Eagles and common date Double Eagles in super grades.
As for underpriced U.S., I don't think many U.S. coins are presently undervalued, really. They're either right where they should be or overpriced. However, there are some U.S. currency notes that may be a little bit undervalued right now, but not by that much. Most of those are right where they should be price-wise, in my opinion. Some of the key date modern varieties of circulation series may also be undervalued presently as people are pumping a ton of money into the older stuff and Silver Eagles. Things like 1970-S Small Date Cents, 1972 Type 2 Ike Dollars, 1981-S Type 2 Cents and Dollars are being a little bit neglected at the moment, so I think they may be a little undervalued right now. Not hugely, but a little.