I'd say Klein, he has the best IV spread out of all of them. Jeorge is another solid option though if you want a more speedy archer, Jeorge is more like a team player instead of a soloing ranged nuke.
For now your team is looking fine and I'd prefer using Sanaki so that you have a balance between magic and physical damage.
+Atk Klein can take out most units before taking any damage. He is a much easier unit to use than both Rebecca and Jeorge, in my opinion. Just keep him buffed beside your Sanaki(if you are using), and have one unit with Draw Back. You will probably be using him to take out most of the enemies' units.
decided to promote klein. i think klein will synergize well with sanaki and corrinF fighting tanky enemies. sanaki with seal def3 to attack first thus lowering its defense. after that klein will finish it with brave bow. corrinF will attack near enemies and lowering its attack(that is if it's still alive). also both of them have hone atk3 so there's not much problem to place him in battle. what's a good B skill for klein to work with this kind of play?