There are already many guides out there that evaluate the usefulness of each valkyrie in Honkai Impact 3rd, but not all of them are necessarily F2P friendly.
Here, I hope to give some pointers on valkyrie farming for new players entering the midgame at around level 45. I will try to get new players into the game as quickly as possible, while keeping in mind the strengths of each valkyrie in the endgame so that less stamina is wasted on suboptimal picks.
Because the Infinite Abyss is the largest source of crystals for F2P players, and thus their best chance at getting best-in-slot gear, the guide will be biased towards valkyries that are good in the Abyss. Every valkyrie that is good in the Abyss can be used to good effect in other singleplayer modes, but not every good singleplayer valkyrie is good in the Abyss. Therefore, I think these are solid mid-game goals.
(For those in the know, this guide assumes Yamabuki Armor's infinite laser is patched out)
Without a reliable mob attractor, Abyss floors take much longer to clear. Even if this does not make the game harder, it certainly makes it more tedious. Do yourself a favor and make VR a top priority. It is by far the most consistent attractor, and is also able to bring different types of CC to the field depending on your preferences. Hope you kept those Colt Peacemakers.
Once you get an attractor, you will need AoE DPS. This area is more involved.
Snowy Sniper is actually fairly weak until SS rank, and even weaker without at least Planck (B). However, at SS rank with a good stigmata loadout, Snowy Sniper's strength jumps to the top of the pack.
Picking SS leaves you at the mercy of the Random Number God, but if you get lucky you will have saved a decent amount of time that would've been spent gearing Valkyie Triumph instead.
Another option for AoE DPS is Valkyrie Triumph. She is much easier to gear, is stronger than Snowy Sniper at this point in the game,, and has healing to boot. However, be warned that she will fall off late game with lower damage, especially since she cannot facetank enemies to get charged attacks off all the time.
Picking VT is delaying lategame strength for a more comfortable midgame.
Next up you'll want a support. There is still a choice to be made, but in my opinion this one is easy.
On paper, Divine Prayer looks worse than Yamabuki Armor. The majority of her healing is locked behind a 75 SP ultimate, her leader ability is limited in usefulness, and her DPS is abysmal until SS rank. However, she is a great pick for two reasons:
First, Yamabuki Armor's shields do not protect against Abyss bleeding, which is the biggest source of damage in the Abyss. Yamabuki Armor's healing relies on being able to hit a big mob of enemies multiple times, but most of your time in the Abyss will be spent fighting just a few high HP miniboss mobs.
Second, Divine Prayer has a much greater capacity for inflicting time lock on bosses. This is crucial because most bosses are immume to every type of CC except time lock/time fracture. One of my favorite ways to deal with Abyss bosses is to time lock with Divine Prayer, switch to Shadow Dash and combo attack 3 times, then switch back to Divine Prayer.
If you are already blessed with good DPS, consider getting Yamabuki Armor instead. She has one of the best leader skills in the game for DPS, and you do not need healing if your DPS is high enough to keep enemies dropping health packs at a fast rate.
Yamabuki Armor could be a good option for those that prioritize VT over Snowy Sniper, as VT helps pick up the slack on healing.
Yamabuki Armor herself also deals much more burst damage than Divine Prayer, making her a much better choice for dealing with Mecha type enemies.
As Abyss temperatures continue to rise, you will want to have a dedicated single-target DPS for boss slaying purposes.
Shadow Dash enjoys an advantage over Miko because it is easier to build a high combo count on SD, her co-op team skill is amazing, and because she is especially good against Wendy. At SS rank with 3 shadows on the field, SD can keep Wendy stagger locked, stopping her elite shield from recharging for a long time.
Investing into SD also powers up Memento, a solid elemental DPS character.
Fortunately, you don't have to choose here as fragments for Miko are obtained in a different manner. Thus, you can farm both SD and Miko at the same time.
It's not a waste to invest in both valkyries, as both are different types and it helps to have a more diverse roster for the Memorial Arena. Doing open world also contributes fragments for Memento and Dimension Break.
Once you have a valkyrie from each of the 4 categories, you can farm the other valkyrie in the pair, or consider the valkyries below:
VC is known as one of the best boss killer valkyries in the game, but a big part of her power is gated behind the SS rank. The valkyries above can fulfill VC's other roles. Still, make sure to buy VC frags whenever they come up in the shop, and consider farming VC frags if you have spare stamina.
If Mko and Memento were not enough reason to do open world activities, then DB should be the clincher. DB is considered to be one of the best supports, because she keeps the time lock that Divine Prayer has, adds an AoE effect to it, and trades in the healing for a truckload of damage. DB is indispensable for any boss killing team, and also works against mobs in the Abyss.