Hey, we have a thread about which cards people hate, so it's only fair 
I'm going to use this little format to classify the different kinds of awesome that a Dominion card can be, feel free to use it or not:
Best Design -- These cards are balanced and interesting. They interact in many and sometimes unexpected ways with other cards, and go up and down in value depending on what else is in the spread.
Most Fun -- Actually using these cards is a joy, even if maybe they're a little too good or not good enough.
Most Powerful Yet Underrated -- The cards that make me happy when I see them, because I'm probably going to win.
Important -- I see these as solving problems, filling holes, and generally making the game go, despite not being uniquely as awesome as the others.
First off, I really agree with a lot of the cards you've listed.
Watchtower is also my vote for "best-designed Dominion card"- it does so many things well- substitute it for Library in a Festival/Hamlet engine, stop Ambassadors and cursers in their tracks, use it as a cheap Royal Seal, beef up your Goons engine, etc. And yet, as you mentioned, it's not actually the most powerful of cards. Knowing when to get one, and how to use it, is really subtle and fun.
Tactician is also up there for me. It's an odd card in that the right answer to "how many to buy" is quite often exactly two.
Horn of Plenty is another one I find myself nodding in agreement with. It doesn't have a great win rate on Isotropic overall, but I've found it to be utterly essential a lot of the time, and it seems to figure prominently in many games with higher-ranked players. Easily the best Workshop variant we've seen so far, and possibly my vote for Dominion card which best rewards skill.
Durations and VP chips are probably my favorite mechanics in Dominion. I love them all, except maybe the noob-trap Outpost.
There are a lot of cards in Dominion which are probably too powerful, obviously and game-changingly powerful as long as there's any support at all, but powerful in really fun ways. You more or less have to go for these cards, but it's fun to do so anyway.
Goons (with Villages),
Tournament (almost all of the time), and
Scrying Pool (with money-giving actions) are these cards for me, also maybe
Menagerie (with any trashing).
There are other cards which are kinda underpowered most of the time, but are really fun to use all the same, to try and create some sort of alternative engine. I don't do a very good job winning with
Trade Route, but there's an excellent tension within the card that I like trying to manage.
Coppersmith is normally a garbage card, but I love the occasional times it's actually useful (for example, as a combo with Apothecary or as a Bridge/NV kicker).
Apothecary is itself great as an alternative to trashing Coppers, and unlike the 'smith is actually a good card much of the time. And, of course,
Gardens. I go for Gardens decks way more than I ought.
Cards which do weird things with costs, either buying or trashing! I love these too.
Salvager,
Apprentice,
Peddler,
Grand Market. All very powerful, too.
But the card I am happiest to see in a random Dominion setup, the card I have the highest opinion of relative to the rest of Isotropic, probably my favorite Dominion card in fact... is
Festival. Plain old base-set Festival. Plus-two actions, plus-two coin, plus-one buy. It's my highest "effect with", and third-highest "win rate given available", behind
Young Witch and
Scrying Pool. It's in my top ten for "win rate with" despite being a card that most people lose with. And unlike Bureaucrat (which I actually win with on the very, very rare occasions I buy it- mostly Gardens and Duke games), I buy it a lot. Like in over 90 percent of games a lot. Festival is my secret weapon.
Why? More than any other basic card, Festival provides an interesting tradeoff- you get money, and actions, and buy, but you lose a card. I've found that the trick to using Festival effectively is to a) find ways to mitigate the loss of card, and there are many of them, not just Library; and b) actually take advantage of the buy. And Festival makes it easy to do that, being the only +Buy card which also gives action and money. Festival is an engine builder, Festival is a card that loves double-Province turns, I love double-Province turns, and man I love Festival.
