Some notable units you have that are really good (going from top left to right each row) are:

• Peony, Harmonic Roy (also known as Groom Roy), Reginn, Halloween Female Corrin, Brave Edelgard, Duo Female Corrin (Ninja Female Corrin), Duo Lyn (Ninja Lyn), and Brave Female Byleth.

• Duo Elise (Valentines Elise) and Gregor.

• Legendary Male Byleth, Seiðr, and Ash.

These units are already pretty good out of the box. Just have them learn the skills they came with and give them a run.

You have some other characters that are pretty good like Legendary Hector, Brave Claude, Reinhardt, Ishtar, Igrene, Brave Eliwood, and Valentines Effie... but they require more investment, so you should save them for later once you get more accustomed to the game.