Here is a short list of what I think have been underrated cards based on the 2017 Qvist rankings. Some of them are Nocturnes, others are old past favorites. Feel free to add or dispute anything on this list.
Knights
This pile has seen a continuous rise over the years, but it still isn't high enough. Knights are devastating when uncontested, and the person who loses the Knight split is racing against the clock. With Lurker, Graverobber, Necromancer and Rogue, games with Knights are more centralizing and degenerate than ever. Don't forget Throne Room variants either.
Tracker
I know, I know looks crazy. Just start buying it. You are going to be surprised how useful and consistent it is. The topdeck ability alone is great, much cheaper opportunity cost than a Royal Seal, and Boons are very useful.
Fool's Gold
Yeah, mass FG is bad. But you know what isn't bad? Great payload at the cheap. This is one people are coming around to as well, and I almost didn't put it on the list.
Borrow
Events in general are still underrated. There is a famous maxim, "wherever you rated the Events, put them 10 spots higher." It's a ludicrous statement at its core, but it shows a truth. Events absolutely dominate games and Borrow is no exception. Think of a game where Borrow is on the board and you never used it. Exactly.
Traveling Fair
Why in the hell is this dropping in rank? It should be going up if anything! Topdecking cards and extra buys for only $2 is insane power. Consistency is the best friend of engines with the exception of the very lucky Tournament deck. If you are not using this Event, you are going to lose every time to someone who does.
Druid
This one is understandable, because Druid by itself sucks. Just more of a PSA that Wisps are the most broken thing ever, and that you should basically open Druid any time there is Swamp's Gift set aside. Yes, even forgo trashing if Flames is included. Other reasons to maybe get Druid: Earth's Gift, Flame's Gift, Field's Gift, +buy.
Doctor
People have gotten too annoyed at Doctor's bullshit to realize just how fast of a trasher Doctor is. I have won games getting Doctor over say, Steward. Not joking. (Speaking of which, if there's an Overrated list ever, throw Steward on there, please.) Just pay attention to your deck contents at all times, swallow the pill, and play with Doctor more. It's really not that bad.
Expedition
Continuing the theme of misaligned Events. Draw is good, handsize increase at start of turn is good, Den of Sin and Wharf demonstrate this, why is this dropping? Consistency is good for engines, and Expedition provides it. It isn't the most powerful Event ever but with the right Kingdom you can play with Expedition support handily in your deck.
Fool
Yeah it's bad, but not basically at the very bottom of the list bad! Fool provides an alternate path to building in the opening over Silver. Most of the Boons Fool provides give economy (Mountain, Field, Forest, Sky), cycling (Sea, River, Wind), or enginey stuff (Swamp, Flame, Earth). The mostly garbage Boons in this sense are Moon, Sun and sometimes Sky. It could go bad but statistically it should be fine. Of course, you get a dead card in Fool if uncontested. If contested, Fool gets a lot better. Time will tell but it doesn't seem to be that close to the very bottom of the $3s.
Marauder
This is a must open on so many boards, and it gives you payload too. People see this and think Sea Hag. Marauder is so much better than the Hag.
Exorcist
Placed horribly currenty, yeah it's Nocturne but it should see an incredible rise. Estate trashing into Wisps is an incredible tempo booster, Silvers become great draw in Imps and if you need it, more expensive engine parts become Ghosts. And it's a trasher you cannot draw dead. Uh sign me up.
Advisor
It's actually really good draw on the cheap. Just start buying them en mass. I know I know you lose your best card but soon you stop caring because you're drawing everything.
Engineer
This has not risen nearly fast enough. Being able to trash your hand and pick up an Engineer is such a HUGE boost to tempo. There are so many cool things you can do with Engineer.
Bishop
And this has continued to drop because.. why exactly? There also seems to be this pretend rule that you are not allowed to open Bishop. Bullshit, you absolutely can sometimes. Yeah your opponent gets thinner, so what? So do you! And you get VP with it, he doesn't. There are boards where the VP doesn't matter and the trashers are great, but with a dearth of both consider yourself a Bishop opening. Look into the terminal space, the kind of deck needed to be built, etc.
Groundskeeper
With enough Groundskeepers, you don't need to buy Provinces. You can buy Estates or Duchies, sometimes for even more points! You can prolong the game as long as you like. Engines love this card. At the worst Groundskeeper ends up being a cantrip Duchy or better. It's a heck of a steal.
Den of Sin
New Card Syndrome applies to Den of Sin as it is from Nocturne, but man what a card. While worse than Wharf, Den of Sin can never be played dead, and on buy you get to use it immediately, leading to such an increase in tempo. It is a very strong draw card.
Triumph
Man, have engines somehow gotten worse in 2018? Why is this dropping? Jsh wrote an article and everything!
Forum
I think this one will be forever underrated honestly. I know, it looks like it sucks. Just start picking it up on $5 if you have nothing immediately better to do. You will start winning tons of games like you couldn't believe. It is a consistency machine. Nothing like Warehouse. It's basically draw.
Legionary
It's a terminal Gold that has one of the most brutal attacks in the game. It's dropping for some dumb reason. I see people skipping it on boards they shouldn't, so maybe that is why.
Council Room
It's just a slightly worse Margrave. Please, start buying this thing...
Jester
Overrated for so long, the worm has turned and now it is finally underrated. Basically don't ever fall behind, because Jester favors the player who is ahead. If you are behind, don't pick it up and keep building, although you're probably sunk in the long term.
And that is all that came immediately to my mind. Let's stir this controversy pot and see what we get.