Eliwood Normal
My recommendation: Nino
There is no slowed rate of experience, so my characters will be high on levels and it won't hurt me to raise her since my characters will still be strong and the enemies aren't particularly difficult either.
Eliwood Hard
My recommendation: Eliwood
It seems odd to give it to him, but he promotes very late in the game, and his route has less Chapters after main lord promotion then Hector's does. You can keep him as a level 15 Lord before giving him Afa's Drops to increase the chance of him gaining well without making it too difficult to raise him either.
Hector Normal
My recommendation: Legault
Legault is powerful enough that he can gain no levels and still be raisable with Afa's Drops. Also, it will make him easier to raise once he becomes an Assassin, an often tedious task.
Hector Hard
My recommendation: Heath or Farina
You get Heath and Farina only slightly after you get Afa's Drops. Even though Heath get's a stat bonus since it's Hector's Hard Mode, it still isn't all that easy to raise him. You also can Arena Abuse him only a few Chapters after recieving him. Farina comes later, has no stat bonus, but comes higher on levels with respectable starting stats. She also turns out stronger, most of the time and you can't Arena Abuse her. For this reason, I give it to her. Still, Heath is a VERY good option to give it to.