So I'm going to assume we mean Curse-trashing after the fact, yes? (This rules out Watchtower and Trader)
These are my opinions. Someone else feel free to take my list and modify it.
GOD TIER
Masquerade: Sends a Curse back to the opponent and can trash one regardless. God Tier.
Jack of All Trades: Gains you Silver, you don't mind using your Trash effect on a Curse, cycles cards. Also God Tier.
Hermit - You can trash from the discard, you can trash and gain new parts for your deck, maybe even a Madman.
HIGH TIER
Junk Dealer - Nonterminal, gives you cash and Curse is the optimal target for trashing.
Trading Post - Not as good as Jack for going Curse-> Silver, but it can trash something else too.
Upgrade - Nonterminal, you want it for other stuff too.
Remake - Can trash 2 at a time, and you can do something like Estate -> Silver at the same time.
Lookout - Obvious Sea Hag interaction, but it's second only to Hermit in trashing things outside your hand.
Forager - Nonterminal, gives you money for it.
Mercenary - Harder to get, but gives you a great bonus in addition to trashing if you have junk.
Altar - Replace a Curse with a Lab? Don't mind if I do.
Trade Route - Not the best, but it's a wonderful late-game trasher once green cards have been bought.
Steward - You primarily want it to trash Copper and Estates, but it can also function as a Curse trasher.
Ambassador - See Steward, only with the benefit of attacking too. Unfortunately, not as flexible as Steward.
Forge - Cost-prohibitive, but you can always trash excess Curses from hand with it.
SERF TIER
Dame Anna - Too much of a crapshoot to get, but if you do it's great for Curses. Sadly, you can also lose it just by playing it if it hits another Knight, so it's just not reliable.
Apprentice - Nonterminal, but it really wants to be trashing bigger things. Not a cantrip like Upgrade and Junk Dealer.
Transmute - If you're in a situation where you wanted Transmute in the first place, it's a great Curse trasher since you will draw it in hands where it can't do much else.
Count - A little unreliable, but worth mentioning. You have to have hands full of junk, which doesn't always happen ironically.
Bishop* (Not counting opponent's Bishop) - Helps your opponent more than it helps you when trashing Curses.
Farmland - Cost is high, but it's a solid move in the late game.
Rats - Only good if you have a use for the Rats afterwards.
Chapel - Chapel is for Copper and Estates, Curses are just icing and you don't want to be in the position of playing your Chapel late in the game if you can avoid it.
Salvager - See Apprentice aside from the nonterminal part. Still better than nothing.
Trader* (Not counting the ability to block Curses) - See Salvager
Develop - See Trader, only with Develop you needed to have a good plan for it in the first place.
Expand - $7 for a Curse trasher is too much; you are buying this for other things and trashing Curses as the gravy on your mashed potatoes.
Governor* (Not counting opponent's Governor) - Gaining an Estate for a Curse just isn't that good. Can still be beneficial if there's a good $2 option, and Governor is of course good for other purposes.
Remodel - See Governor.