Here's my list of units I find underrated: FE7: Lyn- I find her to be pretty good yet for some reason people always bring her up in the "worst lords" type of lists. She's no Ike but I still think she's very dependable.
Isadora- She's got good bases, joins kind of early and is in one of the best classes on the game.
Harken- I guess it's because he joins late that people don't acknowledge him as much as Raven but I've always felt like he was the better of the two.
FE8: Eirika- Sort of like Lyn, I always see her get lumped with the bad lords but I find it more disagreeable when people list Eirika. She gets two rapiers (depending on her mode) and a horse on promotion + Sieglinde. The only thing I find lacking about Eirika is her lack of 2-range weapons because she's very dependable and certain kinds of runs make me appreciate her a little bit more. Saleh- I'm not sure how people feel about him but I don't see him brought up very often. He's really good and in my opinion the best go-to warper in this game.
FE9: Astrid- I'm not sure why she's considered the least of the cavaliers, I've always felt that she was the most flexible and easiest to work with thanks to paragon.
Ilyana/Tormod- People always bring up Soren as the best mage but I feel like these guys give him a good run for his money. Especially Tormod who thanks to celerity can easily outrun Soren with a good bexp dump.
FE10: Tauroneo- He's not available a whole lot but I appreciate his existence when he is. He can actually do his job as a knight and work as a good wall, I think him and Gatrie are the best knights in the game.
Laura- Sometimes the DB can't afford to self-heal and need to get rid of the immediate obstacles, I find Laura to be one of the most dependable healers and she gets to C staves fairly quickly. After Part 1 she's a good back-up physic user but i'd rather use mend/recover on her.
Gatrie- He's one of the few units that doesn't die in two/three hits and is available for a long time. He's a horse-less Titania, what's not to love about him? I find him more dependable than the likes of Nephenee or Soren whom seem to be more acknowledged. Micaiah- Everyone seems to agree that Micaiah is a really good healer, and is usually called the best at it in FE10. Yet somehow that doesn't matter when people list her as the worst lord of the series. If what she lacks in combat she makes up for in her healing then why isn't that taken into consideration?
Awakening: Anna- I'm not sure if she's underrated or not because I'm not too familiar with the Awakening scene, but I think she's pretty awesome and i'd call her the best utility unit.