Jagen's eternally useful for his move and weapon rank.
Sigurd/Quan/Oifaye/Shannan are all pretty busted and hard carry their parts of fe4.
Dagdar comes with really good everything and doesn't ever drop off. He just becomes a capturing machine later on. Eyvel is coded to be unkillable early on in fe5. When you get her back in a lategame gaiden/side chapter, she's still viable as a sword infantry unit with capped speed/skl and high wpn ranks and shit.
Fe6 Marcus is a godsend on hard mode and stays valued due to being an 8 move rescuebot for the long ass maps. He only falls off after midgame because he's so busy carrying you while everyone sans Dieck/Rutger/Zealot is a scrub. (Zealot's basically a second Marcus that comes right at the beginning of midgame. Good wpn ranks, high move, Paladins get all 3 of the weapon triangle, and his bases are fine enough to last. Same utility as Marcus with the move and rescueing. Also you need Zealot for the true ending.)
Fe7 Marcus is good for the entire game, even on Hector Hard Mode. That's the hardest difficulty in the game. Dude's got really good bases and with some minor investment in the form of a speedwings for +2 spd, he basically doubles over half the game and avoids getting doubled by all but the fastest. His resistance stat is so good that he just needs a barrier or pure water boost to tank magic and dodge status staves that show up in droves lategame. People have soloed everything except for the Fire Dragon end boss with Marcus and that's solely bc the fire dragon requires the legendary elibean weapons to kill, or luna tomes.)
Fe8 Seth is busted and people have soloed fe8 with him while rescueing the lord. He's so beyond busted that nobody really catches up to him. Everyone else needs exp to not be a scrub or comes later and isn't as good as he is.
Fe9 Titania is busted. Come on it's Path of Radiance. She starts off being your strongest unit and tapers off to just being a great unit.
Fe10 Sothe is admittedly designed to be a good unit in kind of a shitty class, I won't argue with that. However, he takes forever to taper off to becoming mediocre and that's at ENDGAME when he gets to promote to tier 3 and when knives start to show that they aren't as good as the rest of the weapons. Still, that's like 80% of the time he's playable where he's a good unit.
Volug's one of 4 non-royal(or Giffca) laguz you get that can feasibly get to SSS strike. The others are Nealuchi, Ulki, and Janaff. Ulki/Janaff require a lot of dangerous exposure to do this and Nealuchi involves a lot of careful planning and abuse. There's some youtube videos on this. Volug also is one of your best fe10 part 1 units due to his move, combat, and durability. He even gets halfshift so you don't have the laguz meter as an issue. The other prepromote of note in Part 1 is Zihark, who just shows up and makes Edward obsolete in 90 different ways despite being a furry lover.
Fe11- Jagen still good. Wendell still good. News at 11.
Fe12- Arran still useful. Don't forget that fe11-12 lets you reclass people.
Fe13- Frederick's always useful and the higher difficulties eventually just have the earlygame be "have Avatar or Lord piggyback on Frederick."
Fe14- If you get Jakob, he can reclass into an early paladin. Felicia's reclasses aren't as handy. But both Jakob/Felicia start off as support classes with daggers and staves. They're not bad.
Fe15- There's no Jagen type here. Closest is Saber, the merc you get on Celicas Route and Lukas the soldier on Alms. They're the tankiest units on their respective routes for a good while.
Fe16- None of the prepromotes are bad at all.