Let me try my hand here ...
I guess, a year from now, I will laugh about the stupid things I am saying at this highly unexperienced stage about Adventure cards, but meh, the community would be pretty boring if no one posted wrong statements. Only exception: I did not dare to rate these travellers. It's still overwhelming complicated for me.
I also compared my list to weros and Adams, and added where we differ. Most notable exception: Giant gets all three grades. Where will it be in two years?
Coin of the Realm (B+)
Similar to fishing village: less economy, more reliability. Nice price.
Ratcatcher (B+)
Cantrip trashing with the ups and downs of a reserve card. Can’t compete with upgrade, junk dealer, but at $2, it doesn’t have to.
Raze (B)
Nonterminal trashers can’t be that bad, but both for estates and coppers, there are better trashers.
Nice self-destruction, though.
Amulet (A-) (Adam: C+)
Because of the duration effect it roughly trashes one and a half card per shuffle, which is fine. Great in slogs, nice with feodum.
Caravan Guard C (wero:B-)
Sure it’s a nice-to-have, but opportunity cost makes it a second best choice almost every time. I’d take one for free, but that’s not what key card means.
Dungeon (B+)
Warehousy. Probably better in huge decks, and this is where warehouses shine.
Gear (B+) (Adam: A)
Great BM enabler. Close to „play moat, play two havens“, which can help engines get off the ground as well.
Guide (B)
Counters discard attacks, most notably ghost ship. Rarely a dominant card, but always a worthy buy, when you risk to start with a bad hand.
Duplicate (B+) Adam:B-
This is what talisman should have been: Slightly slower, but so much better at gaining those important nonterminal fives, golds, nobles, Alt-VP’s …
Magpie (A)
Either a lab or a cantrip gainer. A-mazing.
Messenger (B-)
Usually, a weak card. But, sometimes you need that woodcutter. Sometimes, a chancellor is worth it. And sometimes, gaining effects can be fancy. Due to these exceptions, not that weak.
Miser (C+) (wero:B)
We all know those jack/Hermit-games where we desperately need a copper trasher. But, not this one. Unless the board doesn’t offer faster strategys, it’s just ts;db (too slow, don’t buy).
Port (B+) Adam:B (wero:B)
Two villages at 4$, without need of extra buy. That’s a key option in most engines, and so, on most boards.
Ranger (B+)
Two of them draw a card less than two smithies, but to compensate, every other play is good to bring synergic cards together (Spoils/Counterfeit …). With a +buy to boot.
Transmogrify (A-) Adam:B (wero:B-)
It’s not really good at trashing coppers or curses. But due to the „into your hand“-clause it’s the best way to handle estates early on; same holds later for outdated cards, e.g. sea hags after the curse war. This is very strong at $4.
Artificer C+ (wero:B)
Seems rather situational to me. Either you have bad cards and want cheap ones, or you have some draw-to-X, or … yeah, sometimes you have 5 and just want a peddler. But at 5$,that’s not impressive.
Bridge Troll A-
Dare I say that the community mostly underestimates cards that do four little favors? Jack, Junk Dealer, Forager, Counterfeit … I guess it’s the same with bridge troll.
Distant Lands B+
Get four VP chips at the price of 5$ plus one play. That’s a good price, compared to e.g. monument, which is a decent card.
Giant B+ Adam:C (wero:A)
+3$ on average, with a strong attack every other play. That’s nice payload.
Haunted Woods (B+)
The attack forces both players to delay greening, and three cards at the begin of the turn is terrific. On the other hand, it misses a lot of shuffles, and 5$-smithies face a tough benchmark (margrave, torturer …)
Lost City (A) (Adam:B+) (wero:B+)
If I had to rate the strength of a card, I’d put Lost City on B or B+, as the opponents draw is huge. However, this thread is about key cards, meaning that a card changes the board. Lost City totally does.
Relic (B)
The first one is excellent, the next you play is a silver.
Royal Carriage (B) Adam:A-
Reliable Throne Room. Reliability is more than sufficient compensation for the duration, but facing the prominent 4$-5$-gap, this is what I expect.
Storyteller (A-) Adam:B- (wero:B-)
A cantrip that turns silvers into labs, Coppers into cantrips and platinum into … 4 labs. Here it is, the BM engine.
Swamp Hag (A-) Adam:B
It’s not far from an overpowered „+3$, each opponent gains a curse“. Sure, there’s the delay, and the self-cursing isn’t mandatory. That’s why it’s not a clear A.
Treasure Trove (B+)
Very strong at BM, and in slogs. Still, in those enginey days it looks a little old fashioned. Would have been graded A in the base set.
Wine Merchant (C-) (wero:B-)
A woodcutter that offers you a 2$ credit. Imho, too expensive at $5.
Hireling (A-) Adam:B
Once you draw your deck, it’s just a lab. But it’s much more helpful to get to that stage, lucky is who gets it early. And in a long game, it’s irresistable.
Alms (A) Adam:B
Often terrific: E.g., with 5/2, Stables/Milita outperforms Stables/Nothing by far. Also, alms boosts moneyless engines, and it’s game-changing in slogs.
Borrow (B) Adam:A
In almost every game, there will be a situation you’re glad to use it. But does that fit the definition of a key card? I might be wrong, but I look at boards almost the same way, whether borrow is there or not.
Quest (C+)
Sometimes, even in 2015, you’re happy to grab a gold. But usually, with an attack, or a 6card-hand, that’s not a big deal. Discard two curses is way better, but who wants to have two curses?
Save (B-) Adam:A
I don’t like Haven. And Save can be even worse in the presence of down-to-x-attacks. Sure, it gets bought when cards collide, but I won’t buy much more terminals because of this option.
Scouting Party (C+)
There’s two situations you want this: You tracked your deck, and expect a bad hand, or you have two coins left. Both do occur in most games, and so, Scouting Party is frequently used. But it’s never game changing.
Travelling Fair (B)
Great combo with Counting House. Decent when engines lack +Buy.
Bonfire (B)
Burns those coppers at a fair, but not sensational price.
Expedition (C+) Adam:B
Rarely a big deal. And I don’t see how it changes the board. Best when you hit three but don’t want a card at that price.
Pilgrimage (B-) Adam:C
Pay 4$ twice for 3 cards can be a great deal, but it only really works late in the game, when the most work is already done.
,Ferry B+ Adam:A (wero:A)
This is a must-buy if there is a 5$ of which you want to get multiple copies, and also if you want to get multiples of a 4$, and have some source of +buy. Otherwise, it’s skippable.
Plan A- Adam:B (wero:B-)
Trash-on-buy is a very powerful speed-up if you want multiples of a specific card.
Mission B+
Depends on what you can do aside buying: Attacking? Trashing? Remodeling? The more of this stuff you can do, the more it becomes ridiculous to pay only 4$ for a turn.
Ball (C+) Adam:B
Pay 6$ for two 4$-cards? Border Village looks better. However, spammable fours make this deal quite nice. Not more.
Raid (C) Adam:B-
Weak version of masterpiece, with an attack to compensate. The thing is, silver spamming is rarely the big deal.
Seaway (B-)
Get that +buy on all cards at the price of $5 instead of $4? Quite ok - if you need it.
Like many +buy cards, they work at their best if there is no other + buy available.
Trade (C) Adam:B (wero:B-)
One-Shot-Trading-Post, kind of. The problem is, at the stage where you have both 5$ and two cards you want to trash, you rarely want that many silvers.
Lost Arts (A) (wero:B)
Enables weird engines with terminal draw. But even with nonterminal draw like e.g. Stables, it allows tons of payload without the need of villages.
Training (B)
At the price of $6, add X peddlers to your deck, if you have X copies of the chosen card. Yeah, that can be pretty nice.
Inheritance (A-) (wero:B)
Trash three estates, gain three VP chips, gain three copies of a 4$; and for the rest of the game, you can buy at $2 a VP-chip and the 4$ card you’ve chosen. Sounds powerful? It is: If there’s a useful $4, try to get to 7$ ASAP.
Pathfinding (B+)
With X copies of a card, Pathfinding gets you X labs. That can be bonkers, but 8$ is a challenging price point.