I hate Cultist with a passion. Cultist is the only card in the game that is just flat-out overpowered imo. I mean, cards such as Chapel, Page, King's Court and Goons are also strong, but at least they make the game more interesting and they are only part of a good strategy, not a strategy in and of itself.
Other than that, I'm not too fond of cards that give my opponent a benefit, it just feels bad to play/gain them. So that's Council Room, Lost City, Governor and sometimes Embassy.
Unlike most people, I do like Saboteur. It's not very strong, of course, but if you manage to get a Saboteur-centered strategy working, you rip your opponent's deck to shreds and it feels awesome. Same thing goes for Discard/KC/Masq. It's just a different win condition than normal Dominion games, which is interesting enough by itself.