Embargo makes almost every game interesting.
Also Thief. Sometimes it's a serious counter to an obvious strategy (like Fool's gold, IGG or Duke).
I love Procession and Develop too, even if I don't played much with procession right now. They can create very strange strategies.
Young witch is always different with the bane, that's cool.
Horn of Plenty and Bridge for megaturn crazyness.
Knights and Saboteur can destroy the game sometimes, but other times they are also great counters that can makes the kingdom very hard to analyze.
Also, Alt-VP cards deserve a special mention. Fairgrounds, Duke and Feodum first, because the strategy to play when these cards are on the board is not always easy. I like a little less Gardens and Silk road because sometimes the rush-strategy is too obvious, but still, I really like what they add to the game otherwise.
And of course, all my favourites (Menagerie, Island, Rats, Inn, Baron, Coppersmith, Outpost...)