Least favorites:
Governor: Maybe I misplay it, but I often find myself in a point hole that I can't quite make up with the deck I have. I just hate how you pretty much always have to go for it, and it seems to work out for me less often than I'd like.
Minion: Nothing frustrates me more than repeatedly discarding 5 good cards for 4 bad cards.
Page: Too many things about it can make me want to ragequit. Warrioring a warrior is the notorious one, but sometimes it can be something like my Champion was in my bottom 3 cards, so I got the last few curses/ruins right before I played it.
Fools Gold: I mostly dislike it because I ignore it perhaps more often than I should, but it also sucks to get behind early on the split, like if you get a +buy card and it misses the shuffle or something.
Favorites:
City Quarter: I like how the timing works for this card. Sometimes I get it wrong, but it's really fun to get right.
Black Market: Pretty much always lets you at least try to do something interesting.
Encampment/Plunder: Another one that I sometimes play wrong, but it's really fun to get the majority of these and rack up points while drawing the deck.
Farmer's Market: Really fun in an engine game, where hitting the high payoffs and keeping the right number of these around keeps things interesting.