For Conquest like everyone said, Haitaka is very useful with his rally and he can help making it through chapter 10 a lot easier (At least for me because I ended up sacrificing him to survive the last turn). Kumarega is pretty ripped with Magic Counter and Certain Blow from hard difficulty although I never ended up using him apart from MyCastle battles.
And not useful on gameplay but I like getting Nichol and Cadance too only because they somehow have crit cut-ins and I like their design.