It's always board dependent, although mountebank is more annoying early and I would always try to get that first. If KC/TR are around, mountebank becomes even more potent, as the extra coppers are not helpful. If you're trying to empty the curse pile, you need to get one of something else unless you are reliably playing mountebank more than once per turn, otherwise it could take a while.
If familiar is the only potion card and one of witch/sea hag is available, avoid the familiar. I see so many people going for them while I get witches and the witch just wins. Opening 2 silvers (or a terminal $2/silver) gives you a much better chance at a witch than a potion does at a familiar as it isn't relying on 1 card, you just need $5, and the money is useful after the 3rd time through your deck, while the potion isn't. If there's another good potion card, by all means go for the potion/familiar. Sure they end up with more familiars than I do witches... but that's because their only other choice is silver and 2 cursers is plenty, by the time you get 3 or 4, you're lucky if you can play them once each before curses are gone.
The trashing level of a board also matters, if it's good, you may choose not to open sea hag and get witches later, if it's none, it's almost surely right to buy a sea hag with a 4/3 opener.
As has been said, torturer is nullified by the presence of another cursing card, and shouldn't be taken into consideration. It's a beast when there's no other cursing though.
If ambassador is present, you may want to avoid the cursing all together (especially sea hag, ambassador is just better than sea hag, I also avoid sea hag if masquerade is around) depending on how ambassador heavy your opponent goes. Personally, I prefer ambassadors to cursing cards, as they give your opponents useless cards while removing your own. If I'm winning the ambassador race, I will often buy a cursing attack later just to add to their misery as they have too much crap to return and can't keep up.
A lot depends on what +actions are available as well. Witch is much better when they are around, and they help support more than 1 mountebank too.