If you are lacking a damage dealer (because you need at least 2 for Spiral Abyss), Ningguang is a very strong character that can be a main DPS, or a sub DPS. She's a Geo catalyst user that can attack pretty fast, as well as deal huge burst of damage from her ultimate that only costs 40 energy to use. Being a Geo element means that she can only cause a "Crystalize" reaction for more survivability, and also a consistent element to go against most enemies without having to worry about elemental advantages/disadvantages except for going against Geo enemies.
If you already have a good main damage dealer that you rarely switch out of, then Xingqiu is an amazing DPS support. His elemental skill creates rain swords that reduces damage taken, heals you when it gets destroyed, and can consistently apply Hydro element if it touches the enemy. The last part is only good if you are using a melee character, which can make it very easy to cause constant elemental reactions. To top it all off, his elemental burst makes it so that the rain swords don't shatter when you take a hit, AND make it so that your attacks are followed up by sword rains that also apply Hydro. His elemental burst damage is considered to be one of the highest in the game if you are able to continuously attack.
What about the rest? They help with certain niche roles, and are by no means bad. If you already have Ningguang or Xingqiu, and are considering which one to pick out of the others, consider some of the things below.
Xiangling is a free character that everyone will get, simply by completing floor 3 of the Spiral Abyss. This makes her possibly the least recommended character to take. However, she is considered to be a very strong physical damage dealer, so getting a duplicate of her for constellation 1 will make her slightly better, but not amazing.
Xinyan at constellation 0 is mostly just a defensive focused tank, with some damage built into her kit. She can help with applying Pyro element through her skills, but they come with some situational conditions that can be hard to justify getting her.
Beidou is a "tank" with situational conditions as well, except she's capable of dishing out some big burst damage in an instant, rivaling some character's elemental burst damage. She's pretty strong if you are able to make good use out of blocking with her, which requires some good timing and reflex to pull off. Her elemental burst is also very useful for applying Electro for reactions.
Chongyun is viable at both main DPS, or as a support DPS. His elemental skill converts melee attacks into Cryo element, making it easy to cause elemental reactions, and his elemental burst deals pretty good damage. As a support, his Cryo field helps characters like Diluc, Bennett, or Kaeya to deal more damage, and also gives some nice utility bonuses at constellation 2. Being a claymore user also means he helps take down Geo shields, structures, or mining.
Constellation Power Spikes
Certain constellation levels can give a character a noticeable power spike. This can be worth considering if you like to use them, instead of going for another character just to get them at constellation 0.
Ningguang at constellation 6 makes her extremely powerful, but it may be too far to reach for many players. Luckily her first 2 constellation levels are also pretty good. C1 makes her normal attacks into AoE, which helps with fighting enemies in general, especially in abyss, but it won't do anything against bosses, so not really that important. However, she becomes quite amazing at C2. The ability to reset the wall's cooldown can proc whenever you use the skill again, or when you use her elemental burst, or when an enemy destroys it. This can essentially make it so that her wall is on a 6 seconds cooldown if you break the wall by making another wall. Damaging an enemy with the wall will generate some energy, but not if you use 2 walls right away due to some internal cooldown. She becomes an energy battery for herself and the team to reach bursts more frequently.
Similarly, Xingqiu is also very powerful at C6, but his C2 and C4 are also quite nice. At C2, he becomes more self sufficient since he can reduce the enemy's Hydro RES by 15% by himself, effectively increasing the damage of every single sword rain that damages the enemy. But this can be substituted with an Anemo character using Viridescent Venerer artifact set if you happen to be using an Anemo character. His C4 simply increases the damage of his elemental skill by 50%. Sounds simple, but the skill already has higher multiplier, so the 50% can help with one shotting enemies/bosses.
Xiangling's constellations are all nice additions that will increase her damage. But if you are only using her as a support, then you only want to look at C1, C4, or C6 for stronger Pyro damage and longer elemental burst duration.
Xinyan's constellations mostly help if you are only using her as the main damage dealer. At C2, the physical damage part of her elemental burst is guaranteed to crit, potentially making her into a good burst dps if you just dump a bunch of crit damage stat on her. If you are not planning on using her as main damage dealer, then her constellations aren't worth going after, even with C2, her burst damage isn't all that impressive.
Beidou's constellations are nothing to write home about, because they're all just nice additions. C1 gives her a shield if you use her elemental burst for increased survivability. C3 is most likely the one you want to go after since it increases the level of her counter attack skill.
Chongyun at constellation 1 makes him a more reliable main damage dealer if you are using him for that. C2 is good for support, and works quite well with main DPS bennett. C4 and C6 are for main damage dealer use only.