Got a selector box and not sure what to use it on, and want my opinion on what's good?
Note that some selector boxes only give you a choice of operatives up to a certain patch - operatives newer than that can't be chosen.
Want my opinion on what operative to use your selector box on? Click here
(Note: I recommend you pull 1 copy of every new operative so you can use them. That will use most of your pulls.)
If the answer is no, then you can use any girl as your main/dps operative.
When selected as a support operative, Eatchel - The Cub can passively heal you, which can somewhat trivialize the game.
If you don't care about endgame content and doing lots of dps, and you don't already have her yet, then you should get Eatchel - The Cub.
Note that even for endgame play, Eatchel doesn't really receive much benefit from her Manifestations.
Getting her first Manifestation is great, but that only requires 1 dupe or 30 days of collecting her Personal File, which isn't that difficult.
Manifestations M2-M5 don't really help Eatchel that much, and you can generally ignore them.
I recommend you don't pull dupes for Eatchel - just get her first Manifestation from the Personal File, and just leave her at M1.
Eatchel's 5* weapon (Blitzing Fangs) is a great weapon for Eatchel as it allows her to both heal more and passively provide a nice ATK buff, so if you don't care about endgame content, then I recommend you use all your weapon pulls to get Blitzing Fangs.
As far as casual play goes, the only 5* weapons you really want are the 5* weapon for your main dps, and Blitzing Fangs for Eatchel.
Vidya - Celestial Swan is the games strongest support operative by a vast margin.
For endgame content, you should very literally always use Vidya as one of your support operatives.
If you don't have Vidya - Celestial Swan yet, and she's available, then you should ignore everybody else and focus on getting Celestial Swan.
Celestial Swan's 5* weapon (Starlit Truth) is both pretty strong, but is also a massive QoL buff as it makes her support skill last indefinitely, and is absolutely something you should get if you can.
Given that you'll always use Vidya in endgame play, giving her such a huge QoL buff will be very useful for you.
Celestial Swan's logistics set is really good for her, but she doesn't really care about the logistics officers stats - good stats do help, but not that much.
Make sure you get her a copy of her logistics set, but you don't really need to worry about them having great stats.
Note that Celestial Swan is both the games strongest support, and she also absolutely sucks as a main dps, and I don't recommend you ever use her as your dps.
Who to recommend beyond Eatchel and Vidya is difficult.
If you already have a strong dps, then what you'll probably want the most is a strong support for them.
If you don't already have a strong dps, then you might want to pick the one that works the best with your current supports.
If you already have a strong dps, then you can look up that operatives page to see which supports work well with them.
If you already have a strong support operative, then you can look up that operatives page to see who they are good at supporting.
You can also just look at my Quick Reference page to see what each operative does and good they are at doing it.
If you still have genuinely no idea?
Then pick the first operative in this list that you don't already have:
[Universal Support] Vidya - Celestial Swan
As mentioned above, Celestial Swan is the games MVP, and you should always prioritize her above all other operatives.
There are no exceptions to this.
If you already have Eatchel - The Cub and Vidya - Celestial Swan, and already have lots of investment into them, then get Clarina - Crimson Moon.
Eatchel and Vidya are Clarina's best supports - more so than any other operative
As such, if you already have Eatchel and Celestial Swan, then Clarina will work very well as a dps, as she works with the supports you already have.
[Main DPS - Electric Skill] Chenxing - Jade Arc
If you already have Jade Arc, then get Yao - Nightglow.
[Main DPS - Frost Ballistic] Katya - Rhapsody
If you already have Rhapsody, then get Tess - Phantasia.
[Main DPS - Kinetic Skill] Acacia - Chrono Echo
If you already have Chrono Echo, then get Marian - Aquila.
[Main DPS - Thermal Skill] Katya - Rhapsody
If you already have Rhapsody, then get Tess - Phantasia.
[Main DPS - Chaos Ballistic] Bubu - Specular Soul
If you already have Specular Soul, then get Fenny - Starshine.
[Main DPS - Frost Hybrid] Katya - Dawnwing
If you already have Dawnwing, then get Tess - Phantasia or Siris - Freefin.
(Most of Dawnwing's damage is skill, but she shoots so much that Phantasia works great for her.)
If none of those operatives are available, then I recommend you just wait until one of them becomes available.
If you can't wait (either because your selector box will expire or you actually need an operative because you don't have anything), then should none of the above be available I'd generally recommend the following
Siris - Freefin - Buffs skill damage
Tess - Phantasia - Buffs ballistic damage, and adds Frost damage to your shots
Marian - Aquila - Buffs all damage, and deals some Kinetic damage of her own
Yao - Nightglow - Buffs ballistic damage, and deals some Electric damage
Fenny - Starshine - Buffs ballistic damage.
Enya - Tempest - Deals PBAoE Kinetic damage.
Mia - Starchaser - Deals Electric ballistic damage.
Bubu - The Geomancer - Deals Thermal ballistic damage.
Lyfe - Infinite Sight - Deals Kinetic ballistic damage.
Snowbreak has far fewer strong support operatives than main dps operatives, so if you already have two strong dps operatives, then I recommend you focus on getting supports.
You'll almost certainly get a new dps operative with every patch, but it can be a while before a suitable support comes around.