Keeping a Twilight Drake or a Mountain Giant is good in any matchup. Against aggro, you would look for Darkbomb, Defile, Vulgar Homunculus, Spellstone, and Hellfire. Molten Giant can also be kept against aggro.
Against decks you want to pressure, like combo decks or control decks, look for Twilight Drake, Mountain Giant, and Faceless Shamblers.
Keep Eater of Secrets against Mage and Paladin. Keep Skulking Geist against Priest, Rogue, and Druid. If you have the coin, you can try Skulking Geist before Mysterious Challenger comes down.
With the reverted nerf to Molten Giant, it will be back to 20 mana. This list is of course blind to the other cards from Witchwood, but old-school style handlock shows promise with all it’s even cards and the new inclusions like Vulgar Homunculus, Lesser Amethyst Spellstone, and the incredibly powerful Genn Greymane. Eater of Secrets is fantastic cards against Mage and Paladin, who use Ice Block and Mysterious Challenger. Skulking Geist is mandatory in control decks that don’t require 1 cost spells, removing key cards like Inner Fire, Dark Pact, Jade Idol, Conceal and many more.