Aw, what the heck. I like making lists. But really, Torturer and Wharf are easily the two best, and all the rest are pretenders. In general, I like the Labs a lot better than the Smithies.
1. Torturer. Torturer + any Village at all (probably even Shanty Town, ugh) is usually the winning strategy on boards where it appears, since chained Torturers are… well, torture. There are exceptions! More direct curse-givers will short-circuit this plan, Minion with strong trashing can evade it, Gardens just doesn't give a flip, Ambassador is an obvious counter and dominates just about anything anyway. Single Torturer is nothing special, and I'm likely to do something else if there are no Villages and other attacks, but usually there is a Village, and then it's a race to get your Torturers to hit first. Torturers are one of the very few cards that will spur me to actually buy a Moat, which should give a sense of their power.
1. Wharf. So many cards! A mini boost this turn, and a free Smithy plus Buy the next: strongest card-draw in the game*, much like Fishing Village is the strongest source of +Action. Safe to buy multiples even without Villages, and one of the easiest ways to draw your deck with them. And the Buy means it's worthwhile to build a big engine, because you can build up to double-Province even if there's no other Buy in the Kingdom.
*Not counting the upper end of Scrying Pool possibilities.
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3. Nobles. Nobles is great! +2 VP is nothing to be sneezed at, and while you generally don't want to be using your Nobles to give +Action, the fact that it gives you that option makes it much more flexible and valuable. As long as Villages are out (and even sometimes if they aren't, i.e. Scrying pool decks), Nobles are a fantastic way to add VP without diluting your deck strength, allowing you to power through to a stronger endgame.
4. Council Room. +4 Cards is a lot; it's the same net +Cards as Wharf, all in one turn. And the +Buy is also great for giving your +Actions/+Cards engine a reason to get big. But the +Card you give your opponents is a sizable disadvantage. Obviously, Council Room shines when there are discard attacks, but it can be risky otherwise.
5. Rabble. The attack is a lot more subtle and less damaging than Torturer, but it can be quite painful in setups without +Buy, such that you have to green slowly. I'm disinclined to build an engine around Rabble most of the time, but it's a great mid-to-late game buy.
6. Smithy. Whee, the original. Like Envoy below it, its best use is probably as a single or double addition to a Big Money deck, which is pretty good evidence that the most interesting part of most of these Smithies are the things they give you above and beyond the net +2 Cards. Not worth creating an engine without good kickers.
7. Envoy. I have an (ir)rational hatred of Envoy. Most of the time, It simply ensures that you never get to play your best cards, since they just get Envoyed away. Yeah, it's great as an addition to Big Money, but Envoy Big Money games are boooooring. Sometimes, I will look at a board with no synergy, or a lot of cards that benefit by being in an engine and Envoy's the only +Card out there. And I realize "crap, I'm gonna have to buy an Envoy". Then my heart sinks like a stone. But most of the time, I find a way around Envoy, and most of the time that alternative strategy works! Envoy is one of the most overrated and frustrating cards in Dominion.
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ETA: Library and Watchtower are different beasts, but they also kind of fit here, as terminal sources of +Card. Library would be near the bottom of the list, maybe even dead last. Some of the time, it's great: if it has its specific enablers (Fishing Village/Festival/Hamlet), or as defense against hand-reduction attacks. But most of the time it's just an overpriced Smithy that's hard to stack: the "set aside action cards" bit is a really marginal benefit most of the time. Watchtower is a lot better than Library IMO: one less +Card for a cost of $2 less makes it a much better deal, and the Royal Seal power/defense against deck pollution makes it useful in a wider variety of situations. Watchtower is still situational, but its low price tag and extra powers ensure those situations are far more common. I'd probably put it on par with Rabble, maybe even a hair better?