Companion to "Compare the Villages" and "Compare the Smithies," this thread looks at the cards which are by themselves spammable, and give you massive drawing power (I guess that the formal definition would be: +1 action and better than +1 card). The canonical and most vanilla of these is Laboratory.
The cards:
Laboratory
Hunting Party
Alchemist
Minion
Scrying Pool
Nobles
Activated Cities
Cards that could potentially fill this function, but which I think actually can't really, but hey if you want to make the case go for it:
Menagerie
Wishing Well
My comments:
Top Tier:
Scrying Pool stands out for me as #1. The fact that it has a (mild, sure) attack built into it and the ability to cycle through your worse cards is huge, and the relatively low cost really helps it, in my mind. If we're looking for a one-card-spammable draw-your-whole deck, I've never had as much success with the others as with Scrying Pool. Though obviously to really shine, it wants to be paired with some kind of useful Action that also gives +1 Action (or villages + terminal actions, but at that point we aren't talking a hybrid engine).
I don't think I'll hear a ton of argument in putting Hunting Party into the top-tier. The gem of the Cornucopia set, Hunting Party shares the ability of Scrying Pool to cut through your worse cards, and is of course unmatched in drawing that one critical card in your deck.
Minion has the unique property of being more self sufficient than any other one-card engine, providing its own money as well as a mild attack. It's poor in Colony games and perhaps just slightly underperforms to its reputation, but that's just because its reputation is so strong. Clearly top tier, particularly in the absence of trashing.
Mid Tier:
Laboratory and Alchemist, perhaps the closest cousins of any two cards in the game, are both solid, nice cards that just lack the something special of the top tier cards. They need support since they don't provide their own money, they don't filter through bad cards, they don't attack. Laboratory is expensive enough that in the early game it's hard to get your engine going. Alchemist is a Potion card, and a more expensive one than Scrying Pool.
Low Tier:
Nobles has been described persuasively as a Smithy with some extra bonuses. It's a good Smithy. It's a very poor engine all by itself.
Activated Cities are amazing when they work, but the initial period of high-cost Villagery is rough to get through. Very situational, and probably the only card on this list that you are fairly likely to want to entirely ignore in a Kingdom.
Honorable Mention:
I do not believe that you can get an "engine" of Menagerie going. It's a worthwhile support card in some decks, but spamming it is counter-productive.
Maybe some kind of nutty deck with a specific trashing strategy would make Wishing Well into an engine (buy one Coppersmith, one or two Markets and spam Wishing Wells and Coppers)? But not usually, not even if you have robot-like deck counting skills. Not a bad card, but not an engine.