Soo many great cards!
I do love myself some good
Ill Gotten Gains. I really like that it does take some thinking about, but once you understand its power, you have a mini-mind explosion. It is also great that it isn't always a must but, such as when there is trader, or watchtower, or other cursers on the board, so you actually have to think about whether it is a wise strategy in the kingdom or not. That makes for a great card.
These are in no order of preference, but I also agree with the cards
City,
Tactician,
Cultist, and
Duke. All awesome cards. Those city stack smacks make for satisfying turns, and as Theory stated on the dominion blog, it is very nicely thematic, which adds a nice little bonus. It is very cool to see that Cultist evolved from a cool card, from Inn, in Donald X's Dominion outtakes (under 'The Early Days' section, +2 cards, you may play another Inn) into an excellent and fun junking attack. Is is great that even when the ruins run out, you are still happy t have them in your deck, with the possibly of crazy card chains!