You want a wall of text, you'll get it.
also, Len's AW passive does stack with Katie's. same goes for Uzume and Aria's.
Olivie: best duelist against physical damage hands down, 100% dodge with relatively decent uptime and downtime. (skill duration vs reuse timers)
Elizabeth: There will be times when you need a good physical burst damage. there are many cases where a VH will be more useful than simply having bashira + spica. look at base scramble I or this video for more info: https://www.youtube.com/watch?v=pqZQvcnnlBQ
Imelia: raw stats make her great: http://aigis.gcwiki.info/?%A5%E6%A5%...C8#unit_bandit she might not have the highest base attack/hp among bandits, but she does have the best def and her skill increases atk and def, and with 3.5k max hp, you can quite easily get her to 3k hp without much effort (and her AW passive adds bandit hp). 3k raw hp can help soak up a lot of damage when your healing might not be enough. I'm using gcwiki link because comparison is easier on that page.
waltz: waltz is the dancer with the highest attack atm, having one as emergency might be good because dancers really boost units like elizabeth and bashira a lot (+300 attack during skill). you might not be used to her, but when you AW 70% of your units you can start trying different things and you can see how good dancers can be when used right.
Saki: black ninja = instant skill, and her attack speed/damage is pretty good for a ranged melee unit that you're going to need regardless.
Horus: high attack ranged melee, means she can do much better off skill against armored units.
Charlotte: multi-attack ranged melee, means she's good at clearing small fries pretty much. her skill has a short duration but a shorter reuse timer as well, compared to horus. either one of them will be fine, but you do need at least 1 good ranged melee and having more doesnt hurt. I'd only go with Saki-Horus or Saki-charlotte though.
Len: useful when AW for 30% reuse timer reduction as well as 5% def passive. that 30% reuse is pretty good for some units that you skill often (olivie, soldiers). I've used her in subjugations and she can be used just fine for soldiers to generate UP faster. and she gets better whenever we get the patch that changes her +hp affection bonus to +attack, which pushes her attack to 300+ when AW. I've combined her with waltz before to great effect.
Shuka: not very useful aside from subjugations which let you start with plenty of UP to place her down, but having an advance guard for the UP cost reduction when you need it is good. Here's a sample from DMM, note that advance guard cost has been reduced there: https://www.youtube.com/watch?v=dZftWXnb8gU
drop rate boosters speak for themselves. they're only there to boost drop rate, pretty much
Shiho: AW for slightly faster attack speed, and you're already using her anyway as a witch.
yuyu: you're going to need at least 1 AW witch, and preferably 2. shiho+ yuyu can mostly cover that, and yuyu is a nice low cost witch with good dps. and as mentioned, she has good synergy with olivie (due to -1 cost as elf)
fedora: low cost healer that is more powerful than alissa, and her range up skill might be useful for supporting heals within another healer's 'territory'.
solano: she is a good dps.
Rikka: she can't be healed, but her raw stats allow her to usually duel a strong mob without too much issue. her high stats also let her soak up some damage from your tanks.
Ertel: melee slot healer, there will be more and more maps in the future where they dont provide good ranged spots (Rita event G-tier map is an example, although you can do without her), but she does open up opportunities as well as support healing when your ranged healers dont heal enough.
Bashira-odette-claudia is relatively self-explanatory. their AW passives are rather useless as compared to the other units, which is why they are mainly AW'd for their stats, which pushes them down priority a lot. the stats bashira gain from AW can also be covered by having waltz support her btw.
Lynn: you never know when a monk might be useful, this is for opening up options.
Julian: his AW is pretty crap, but his SAW is decent (auto-skill) and it's good to have a 2nd UP generator always.
Rita: stealth is niche, but they CAN be useful and it has been proven in Imelia event where their stealth allow them to be placed at ranged spots that would be attacked by pumpkins otherwise.
Momiji/Sakuya: either one really. I dont feel like comparing them, and samurais are pretty niche, but they can be used to take out 2~3 gob archer/mages that come at the same time, as well as be a hp soak. I've used momiji here (Imelia event G-tier): https://youtu.be/NSLDPMbPPbs
1) is obviously the most useful, since they're just outright good units that you can put to use even before AW.
2) is after, because having ranged melee is seriously useful. the imelia event G-tier map above also required a ranged melee unit.
3) & 4) are relatively similar in priority, but I do want Len for 5% def.
The rankings are just general priority and can be switched around however you like it. you just need to work on them all eventually. some of them are mentioned even though you cant AW them yet (like solano and rikka, unless you want to use a gold monk for rikka), but that's for you to level them to max. you should be leveling units to max at this point to prepare them for AW. leaving them at cc50 is mostly for when you still can't max them and need the resources to focus on leveling other units, which you are over by this point.