shark_bait's 10 Cards to Victory
This is not a list of the 10 best cards in Dominion. Instead, it is a list of the 10 cards wherein fundamentally understanding their impact to a Kingdom will help pave your way to victory. They are the cards that should make you happy because you know that your greater understanding of that card will allow you to outplay, outthink and outwit your opponent. There are 3 main categories that these cards fall under including cards that relate to VP, cycling/deck acceleration and 3-pile control. Obviously other cards in Dominion fall under the umbrella of these categories but this is an arbitrary list of 10 cards that I chose and the concepts that they cover can be applied to other relevant cards.
So here is the list.
1.) Stonemason
2.) Fairgrounds
3.) Vineyards
4.) Possession
5.) Tactician
6.) Scrying Pool
7.) Hermit/Madman
8.) Apothecary
9.) Goons
10.) Platinum/Colony
Don't try to read too much into the order of the list. Just accept the contents of the list and that all of the cards represent an idea that a full understanding of will allow for you to win more games of Dominion.
1.) Stonemason
(http://wiki.dominionstrategy.com/images/d/d2/Stonemason.jpg)
Stonemason is the king of 3-pile endings. Stonemason a Silver into 2 Stonemasons and buy a Stonemason overpaying $2 gaining 2 more Stonemasons to gain 5 Stonemasons in a single turn. Use it to turn a $6 Gold into 2 $5's. Turn a $5 into 2 $4's. This is all combined with the fact that if you can draw your deck completely you can then use your newly gained cards (which could be a Stonemason) to do even more on your turn. Games can end very quickly with Stonemason. Always be sure to think about what 3 piles are close and how Stonemason could be used to quickly deplete them.
2.) Fairgrounds
(http://wiki.dominionstrategy.com/images/7/7e/Fairgrounds.jpg)
A relatively straightforward card in principle but it is quit game changing. This card is a huge boost to engines as a BM player can no longer hope to get half the VP. This card allows for engines to build longer and then capitalize on later in the game with a stronger deck. When this card is out you should think about how many points your Fairgrounds could be worth. Think of how your deck will be build. You want diversity but you don't want to overterminalize such that your deck can't generate $6. Dark Ages helped Fairgrounds immensely with the addition of 8 unique cards. That is almost 4 additional points per Fairgrounds. When using Shelters/Ruins with Fairgrounds think carefully about what you trash. At the end of the game you may find that you should have kept your 3 Shelters to allow your Fairground to go from 6 to 8 points.
3.) Vineyards
(http://wiki.dominionstrategy.com/images/c/c8/Vineyard.jpg)
Another VP card in the list. What makes Vineyards great is that you get points by building a huge engine. Like Fairgrounds, it is a scaling VP card that prevents BM from obtaining half of the VP. Vineyards benefits immensely from cheap cantrips (and cantrips in general) and also from the price point. Costing only $P allows you to use a +Buy in the Kingdom to gain both a Vineyards and an engine component. You can also trash all of your treasure and still afford them. In a Vineyards mirror winning the split often results in Victory. Look for an opportunity to purchase a 2nd or 3rd Potion to try and get a 5/3 advantage. This will force your opponent to find VP elsewhere whereas you can continue to build action cards to gain points.
4.) Possession
(http://wiki.dominionstrategy.com/images/3/3b/Possession.jpg)
I wrote an article here (http://forum.dominionstrategy.com/index.php?topic=10653.0) outlining many of the nuances of this card. What Possession does is flip a Kingdom upside-down. I won't go into huge detail here but when you see Possession you have to be able to recognize (a) Is this viable to play (b) how do I build my deck in the mirror and (c) if my opponent goes for Possession how should my play respond assuming I don't want Possession. Be careful of what cards you put in your deck. Trash for benefit greatly help the Possessor so incorporate them with care.
5.) Tactician
(http://wiki.dominionstrategy.com/images/e/ed/Tactician.jpg)
There are a couple of things about Tactician that warrant its inclusion on this list. First, the idea that 1 big hand is better than 2 normal hands and second is that deck cycling is extremely helpful in deck acceleration. Whenever you play Tactician it is obvious that you lose a turn and that next turn you get a double hand. You also get 5 cards of cycling which allows you to get to your newly purchased cards faster. One large hand being better than two small hands is crucial for hitting certain price points and cards that benefit from large hands. Of special mention is Colony/Platinum and Tactician. One Tactician play will often result in an early Platinum which can snowball into more Platinum. One Tactician hand with Forge allows for a quick and efficient trashing of a large part of the deck. Tactician also has many combos ala the "Double Tactician" deck wherein a Tactician is played each turn and cards are bought using virtual $$$ from Kingdom cards. This is the most "cute" of the Tactician uses but it should not overshadow the others. Tactician is a very versatile card and understanding its use in a variety of situations is essential.
6.) Scrying Pool
(http://wiki.dominionstrategy.com/images/7/79/Scrying_Pool.jpg)
Scrying Pool is almost always good. It is very comparable to Vineyards in terms of what kind of Kingdoms it excels in. You want an action dense deck and cheap cantrips allows for massive card draw. There are few kingdoms where Scrying Pool can be avoided. The attack can be very powerful in both keeping junk on top of your opponents deck and also forcing him to miss plays of key cards in his deck. This attack slows down your opponent giving you more time to build your engine. In general engine with attacks are significantly better than engines without due to what they do to non-engine opponents and the relative value of each players turn. Game extension of 4 turns for a non-engine players gives them an extra 4 mediocre hands that may get a Province or Duchy. However those 4 turns for the engine player come when the engine is running at peak performance and instead of 4 mediocre turns the engine player gains a significant advantage.
7.) Hermit/Madman
(http://wiki.dominionstrategy.com/images/8/8e/Hermit.jpg)(http://wiki.dominionstrategy.com/images/1/19/Madman.jpg)
These are an extremely good pair of cards. Most people are aware of the Hermit/Market Square combo as a game-winning strategy but these cards are more than that. Hermit is exceptionally good at ignoring all Cursing attacks. Being able to trash from the discard allows you to both fill you deck with Silver while simultaneously trashing Curses as soon as they enter your deck. I would go as far to say that Hermit should be bought in every game. It is so easy to replace starting Estates with Silver. That greatly increases your deck's ability to get $5 power cards. You can then continue to gain Silver or $3 actions or you can choose to gain Madmen. I would compare Madmen to Tactician except that you get your large turn on the turn you play Madman. The catch is that you only get it once. Use Madman at the beginning of the game accelerate your cycling and deck improvement or use it end-game with multiple on a single turn for a game-ending megaturn. Look to turn your Hermits into Madmen when possible, especially if you can do two at once. These cards both have uses at the beginning of the game and in the endgame.
8.) Apothecary
(http://wiki.dominionstrategy.com/images/6/69/Apothecary.jpg)
Given the choice between Apothecary and Alchemist, I would take Apothecary even though it is $2P and Alchemist is $3P. Before you start to edgecase me one this I will say that the first purchase with a Potion should be Apothecary. Apothecary provides both deck cycling and hand improvement. This allows for decks to power spike much faster by using newly purchased cards faster. If Apothecary is the only Potion card you should highly consider trying to incorporate into your strategy as the benefits are immense. It has minor combos with top of the deck interaction cards but its main power is that it is the cycling/deck acceleration that it provides.
9.) Goons
(http://wiki.dominionstrategy.com/images/e/e2/Goons.jpg)
Goons is similar to the VP cards included in this list in that BM can't end the game with half of the traditional VP. The iconic Goons games are those where you get around 5 Goons in play on a single turn with a large amount of +Buy and then go on a game-ending megaturn amassing huge piles of VP tokens. However most Goons games are not like that. Lack of Village support, lack of draw and lack of trashing come rear their ugly heads and prevent a game ending megaturn. Outside of the Goons megaturn is the high-skill, highly tactical Province game that contains Goons. If you ignore Goons, you will lose and if you ignore Province you will also lose. In these games you need to recognize the pivotal point in deck development where you transition from deck improvement to score improvement. There is no easy formula to define that state. What you are looking for is the time when a non-VP card will not contribute to earning its equivalent points had it been a VP card. This is where you prioritize Duchy over more Goons. You start buying Copper and Estates with extra buys.
10.) Colony/Platinum
(http://wiki.dominionstrategy.com/images/b/be/Colony-new.jpg)(http://wiki.dominionstrategy.com/images/4/42/Platinum-new.jpg)
One could simply say that the addition of colony and platinum add more points to the game and lengthen it allowing more skilled players to have a greater opportunity to demonstrate their skill. That is true and I don't want this description to take away from that true statement. Colony and Platinum are so much more than that. Normal Province games in a general sense go 14-16 turns. Moving up to Colony/Platinum puts you around 20-22 turns. In addition to having more turns the relevant and important decisions within those turns increases. VP vs. Treasure buying becomes much more important as the game can end with the depletion of the 2nd highest VP pile. I have won numerous Colony/Platinum games because of opponents who bought Province too early instead of Platinum. On the flip side I have won numerous games due to opponents buying Platinum when they should be buying Province. These games make you think more critically about your deck state. How many times will you see a given card before the game ends? Can you start rushing Provinces and reliably pile it out without stalling?
Conclusion
Obviously there are tons of nuances and combos not covered in this broad article. The goal is to highlight key cards that illustrate important concepts of understanding Dominion at the highest level. Thoroughly understanding these concepts and knowing when they apply is not easy. It takes lots of skill and experience to recognize the optimal decision. You win at Dominion when the game ends and you have the most points. These cards and concepts described will allow you to put yourself in a position that gives you the best chance the favorably end the game with more points than your opponents. Whether it comes from quick deck cycling, 3-pile endgame control or tight control on tactical decisions you can never be 100% sure of a win. You can only apply your knowledge as best as you can in the circumstances that you are given to maximize your chances for victory.