Who to promote next?

Rein all the way. DB3, Lancebreaker 2/3, any Cavalry C skill you want if you have any horsey emblem units, otherwise just spur attack 2 for now. If you have Quickened Pulse (you should, from the recent Tempest Trials) then put Moonbow and that on him and watch him get you arena defense points. When you get a 4 star + atk IV of Rein, promote and merge them and make an Uber Rein. Save your feathers on the other 3 probably. Eirika's just ok, Wrys is not worth the feathers for what little he gains at 5 stars, Nowi can have better Iv's, and really takes QR3 to make properly, so maybe just hold off on that one. You'll find better units to promote besides Rein before too long.