If I were to go with genre, I think most error coins are on the over-hyped side. For specific coins, I have three. I do not understand the current fascination with Libertads or modern struck Peace and Morgan dollars. On the flip side, what do you see as underappreciated? I do not feel knowledgeable enough to answer that one. If anything, I am surprised that silver dollars don't generally have much premium to them while ASE's seem to regardless of spot price. The current spot price appears to have eroded the premium for Morgan and Peace dollars.
Lots of over hyped coins. I'll go with the underappreciated. And this is based on me becoming a coin collector in the late 1960's. Wheat cents were the thing. Anyone I knew that had any fascination with coins were excited to find a wheat cent in their pocket. And that was only ten years after their demise..... I never understood why that fascination didn't translate to the memorial cents when they stopped in 2010. I think the entire run of memorial cents is under appreciated.
Other than "errors", to me toned coins are over-hyped. Some of them are butt ugly unless you catch the light just right, but people are going gaga over them. Underappreciated is personal taste for me. I love hub transition varieties - there's a ton of them in the Barber series and some quite scarce - but almost nobody else cares. Which is fine.
