Ignore the bad members (Leo, Fiona & Meg) and the glass member (Edward). You need some tanks to continue. (Leo is only good in chapter 3-13 as a placeholder)
Nolan is high leveled and is a pretty good character.
Aran is a great tank.
Sothe & Volug are both good character because of their high level.
Jill, Zihark & Taunereo are decent characters for part 1. Try to level Jill because she can be good in further chapters.
If you are in part 3, DON'T recruit of the Dawn Brigade a.k.a. Dain when you play as the Greil Mercenaries
Part 1-Prologue: Train Micaiah. Don't waste time with Eddy or Leo
Chapter 1-1: Maiden of Miracles: Use Nolan. Try to incorporate Micaiah.
Chapter 1-2: The Dispossessed: Use Sothe & Nolan. Both good character. Sothe can tank a lot. However, Nolan doesn't. Kee your eye on Nolan. If you can, try to use Laura & Micaiah
Chapter 1-3: A Faint Light: This is the most difficult chapter.
You need to protect not only the civilians, but also Eddy, Leo & Laura. Because it is game over if 1 dies.
Ilyana: good character. However, useless in part 3.
Aran: Recruit him. He is a great tank. He is like the opposite of Neph. Instead of Great speed, he has great defense.
Chapter 1-4: A Distant Voice: Not that hard. Use Nolan & Aran as walls. Use Sothe for killing. Use the mages for distance attacks. Ignore Meg. I didn't bring Leo or Eddie with me from now on.
Chapter 1-5: The Lost Heir: Now you have 2 great characters (Sothe & Volug), 2 good characters (Aran & Nolan) and some mages as back-up. Again, Meg is useles in this sand. This chapter is pretty rough. Look out for the mages.
Chapter 1-6.1: Raise the Standard: Now you have 5 great characters; Zihark, Tauroneo, and Jill joined. Jill can be somewhat vulnerable in some places.
Chapter 1-6.2: Raise the Standard: Foina is low-leveled. Useless
Chapter 1-7: A Gathering Hope: No that hard of a chapter. You can train your character by this boss if I recall correctly. Muarim is a BEAST. He rival Cainaghis if you train him. If you don't, useless after part 1.
Chapter 1-8: Glory Unwanted: Difficult chapter. But you will be fine with Nailiah and the 2 beasts.
Chapter 1-9: One Survives: Try to train Micaiah.
Part 1-Endgame; Daein, Arise!: Easy chapter. You have OP units
Part 2 is just:
Haar & Kieran
MODEDIT: yo use the edit function next time