It's gotta be Spear Master or Paladin.
Hinoka can really use Marriage Seal with Silas to help her HP/Strength/Defense growth. Once level capped, she can do anything with a Lance really. She can stay a Paladin/Great Knight or use Heart Seal to get to Spear Master or back to a Falcon/Kinshi Knight. It is really whichever the army needs. Out of the 3, while Kinshi Knight gives a ranged weapon option, it has low Strength Cap, and it has low Spear weapon Rank. So it makes more sense to go with Spear Master for S-rank in spear. Paladin is good for weapon triangle control. Great Knight is difficult in Birthright without a source of Wary Fighter skill.
As for marrying Silas, Hinoka synergies with Silas really well. Their Support bonus for each other is precisely what each other needs. During story line chapters, and before Marriage and the use of Marriage seal, Silas can shelter Hinoka against Yumi's. Keeping her as a Sky Knight for a small while helps ferrying troops, which is useful for Chapter 10 Ninja Village. So having Silas' Cavalier to mobilize and shield her whenever necessary while recking up Support Points is really nice. Silas doesn't need anyone to reclass for growth. He stays a Paladin just fine as a final class but needs Wary Fighter skill for General Class. So the Marriage doesn't hurt Silas in anyway. The resultant child is great.
So there is only goodies from reclassing into Cavalier and Great Knight and no reason to stay a Sky Knight/Falcon Knight past Chapter 10.