It's been a long while since anyone updated or wrote a review on Agnes, and there sure are a lot of new cards to consider. If you're playing with a limited pool of cards (Base Set and Dunwich) then those old reviews will be very relevant to you.
But if you have some of the later sets, such as Stella Clark or Edge of the Earth, then this review might be of use. Let's unpack which cards remain useful and which have fallen by the wayside.
Agnes' Role
As for mystics that can act as a primary fighter, Agnes is no longer the only game in town. Players can consider using Akachi, Lily, and Dexter without contorting their deckbuilding. But Agnes remains the most combat-oriented of the bunch and that's because her innate investigator ability is to dish out testless damage. Her ability to finish off a boss with only 1 health remaining can mean the difference between which resolution is reached in scenarios. It can also pick off all kinds of pesky aloof enemies, from Whippoorwills to Rookie Cultists.
Triggering her Innate Investigator Ability
The cards that help trigger her power have not changed. The classic remains Forbidden Knowledge which benefits your economy. The supplemental choice is still Painkillers. Shrivelling sometimes triggers it as well. If you're using cards that manage the chaos bag (Premonition or Dark Prophecy come to mind) then you have more control over whether or not the extra damage is triggered.
The only new addition here is for decks that want to dabble in bless/curse tokens, in which case Spirit of Humanity (2) is a way to both load the bag with tokens, while also providing a way for Agnes to deal testless damage while also providing an emergency option to heal. If you run a suite of spells like Armageddon all the better!
Managing Horror and Health
Agnes' testless damage comes with the cost of self-inflicted horror. Peter Sylvestre remains the King of Horror Soak after all these years. and his experienced version that improves Agnes' ensures that he'll likely have a long reign.
The issue with including Peter is that he competes with the Arcane Initiate for the ally slot, and his ability to offer a free search per turn is pretty amazing. Charisma is one solution. Or partnering with a seeker investigator who can offer something like Old Book of Lore. Rare is the deck that has neither of these gems.
For extra horror soak, the classic Holy Rosary remains popular, moreso for the boost to than for anything else.
The only new-ish healing cards worthy of mention are Earthly Serenity (which unfortunately competes for scarce Arcane slots) and Bandages (slotless, but requires playing in advance).
It's also important to mention that Deny Existence is an amazing card for staying alive, sort of like a versatile Dodge that mystics can access. But it's not going to help with the self-inflicted horror that's inevitable when playing Agnes.
Other options exist, but hardly. Activating Grimm's Fairy Tales is conditional. Mister Pawterson uses the already-oversubscribed accessory slot. Clarity of Mind requires the already-oversubscribed arcane slot and 4 actions to heal 3 damage. Fearless doesn't move the needle much either on passing tests or recovering much horror for the deck space.
Managing Enemies
The release of Brand of Cthugha has helped Agnes quite a lot with her combat suite because it's cheap and has lots of charges. Plus it never wastes charges, as you only spend then after a successful Fight test. By the same token it won't damage allies if you don't engage the enemy.
Another card that has changed the landscape is the now-ubiquitous Sword Cane. It can be used for evasion as well as a fight test that utilizes , making weapons like Fire Axe and Baseball Bat less appealing. Even the once-popular Meat Cleaver seems outmoded, though the ability to regain sanity might merit some consideration.
The fight events Spectral Razor, Storm of Spirits, and Drain Essence are all great options. They help conserve charges on arcane assets and they trigger free card draw from her signature Heirloom.
If you're not using hand slots for something like Ritual Candles and are looking for modern solution for those 3-damage enemies you can invest your experience in Cyclopean Hammer. Solves all of your worries about charges, pesky enemies parking on clues, and probably lots of other things.
I don't expect to see Armageddon outside of specialized decks. But Azure Flame might remain because Agnes is able to get some mileage out of the combination with Unrelenting.
Long Shot should prove quite popular for Agnes. Though it doesn't contribute any icons to the skill test, anything that adds damage is a welcome addition for non-guardians who manage enemies.
When it comes to evasion I don't think modern decks are likely running old standbys like Mists of R'lyeh, Blinding Light, etc when cards like Sword Cane and Manual Dexterity offer much more versatility.
Finding Clues
How much of your deck is devoted to clues varies wildly with Agnes. A dedicated protector version might have no clue-finding cards at all. A true solo deck might pack just a couple of the best cards, while a generalist build can conceivably devote quite a few cards.
The standout here is Sixth Sense, which permits Agnes to investigate using her high stat and never runs out of charges. Sometimes it can benefit from the connecting location having a lower shroud, or finding the clue there. Truly a staple for Agnes.
As for other good options Flashlight is amazing against shroud 2 or 1 since only the token fails (negative tokens can't reduce the final value below 0). For high-shroud locations it's hard to beat Read the Signs. And given her resilience, Drawn to the Flame is a perfect card to find a couple of clues and skip the chaos bag entirely. If you're willing to invest some of your hard-won experience, then Divination will prove to be effective.
Most of the other options have seen their stock go down: Rite of Seeking is pricey and charge-limited, as is Clairvoyance. Dowsing Rod is moreso for Amina or Dexter who can manage doom better.
Managing the Encounter Deck
Ward of Protection remains a staple. Given Agnes' great , A Test of Will operates like more wards that involve a skill test instead of inflicting horror.
If dealing with the encounter deck is something you want Agnes to invest in, consider Scrying (3), Parallel Fates or Stargazing. At low player counts the new Antediluvian Hymn looks to be well worth the curse tokens.
[edit: added mention of Spirit of Humanity, thanks @liwl0115 ]
What Would You Highlight For Agnes in the Present Card Pool?