Let's see...
Sain's actually good.
Kent's meh.
Wallace requires you to gimp your lords, as well as go against Lloyd.
Lowen's...
Marcus is a quality Oifey.
Isadora.
Harken's only selling point, if you're using Raven, is Brave Sword. Even then, though, he's a pretty good pre-promote.
Heath. He can turn out, and most likely will, but the fact that you get him so late, and the fact that you'll have reliable fliers at that point, puts a damper on his usability.
Vaida's there if literally none of your other fliers are good.
Oswin. This guy is a hero. One of the consistently good units in FE7.
Bartre, He's better than Dorcas, but that's not really saying much. He suffers from GBA Fighter Syndrome.
Wil's terrible. Just... Ugh.
Florina is a destroyer of worlds. Provided she gets strength. IIRC, she has the same STR growth as Lyn, 40%?, so she's prone to getting strength screwed, but what pegasus knight isn't?
Fiora is Kent on a pegasus.
Farina? Isn't she a mercenary? Anyway, she's essentially the opposite of Florina. She gets strength much more consistently than her sisters, but her speed and skill stats are sub-par in comparison.
All in all, I think I'm going to give it to Oswin. Discounting Lyn's Story, he's available for almost the entire game, and the fact that he starts of on level... 13? 11? makes him strong for the early game. Especially so if you choose to promote him before level 20. Some downsides would be his movement, duh, and his speed, duh. If you can work around those, tho, he's one of the better units in the game.
Now that I thin about it, it's odd that I never used this site before. I've been a fan of the series since '05, but I've never used this site before. I guess I need to start exploring the Internet more and find out what else I've been missing out on. But, I'm rambling. All you need to now is that I'm new here and I hope to get along with everyone.