won't describe Sophia/Wendy as they've already been said
just want to expand a bit on fe14, but if you grind/dlc then everybody's viable of course
BR- well technically everybody in birthright is usable because the game's piss easy but for overall worst you'd probably have to go with Setsuna, as Takumi exists. Mozu has a special mention for also being released to Archer and being level 1 so you'd need to feed her exp, but again you've got Takumi. Hayato's also joins level 1, and iirc you already have an Orochi by then, so there's that. Azama isn't that great either, from my experience.
CQ- obligatory Odin mention, even if you reclass he has an ass E sword base and can't really double anything. He's pretty prone to getting RNG screwed but if not, Astra's deadly. Use Nyx.
Again, you have to spoon-feed Mozu some exp on a lot of chapters, and it's Conquest and Mozu is level 1 with ass stats and joins when things start picking up. As an Archer, is okay. Both of these guys need Heart Seals though, which can be costly and can take up resources so be wary.
Honorable mention to Laslow for sort of being mediocre outside of being a rally bot, and Arthur for really bad Luck so it might feel like FE6 again with accuracy troubles.
RK- Any unpromoted unit that joins in chapter 16+ is sorta bad and kind of not really useful except for Niles, maybe. Odin is super bad for being a level 12 dark mage when there starts to be promoted enemies around. He's no Nino, so I wouldn't waste my time.
(about Subaki, mine, while being sort of useful early on started doing 0 damage late game and just became a staff bot. he still remained in a slot in my party because Hinoka was a pair up bot.. and Hinoka would similarly do 0 damage to things. but in BR go wild)