As it concerns counters to Cultist and Mountebank, I think Trader is one of the best (along with Lighthouse, Watchtower, and Moat). Am I mistaken? Trader isn't going to let you make an amazing engine in the presence of junking attacks (exception: with Storyteller), but it does make it much easier to grab Provinces. I expect that I would still want to get a Mountebank in response to a single Trader from my opponent, but I doubt I would want a second or third. Trader might be an even better counter to a Cultist stack, but I don't think I have any experience with these two cards in the same kingdom.
Hunting Party is both a counter and support for Mountebank. I think I would usually want only one Mountebank if Hunting Party is in the kingdom (with the goal of getting enough Hunting Parties to play Mountebank each turn and buy a Province).
I agree with the crowd that Mountebank and Cultist (in some order) are the best 5-cost attack cards. Good 'ole Witch deserves some consideration for the third best five-cost attack. Yeah, Torturer and Ghost Ship can be more oppressive on occasion, but it's almost always worth it to get a couple Witches ASAP when they are in the kingdom.