Since everyone talked about Anemone, I'll add her to my dream waifu list after looking at her abilities.

Not a math person much - I usually don't look at stats

it not a idea many people will do, but something i am doing.
It might seems really expensive to do this but really it cheaper IMO, but is just slower.

Firstly get one of your knights to level 70, one your NOT going to give any ampys to, or at least not planing to for along time.
this is purely to make sure your helper squads are max power to help you.

Then stop leveling the girl you want to give ampys to, at least 10 levels below their max level (for me it was Black Baccara at level 58)
Then give her ampys to level up, all the ampys I gave to Black Baccara, and she is currently level 68
Doing the maths to work out how many it is 34x HP ampys, 57x attack ampys and 48x defence ampys so far
Also note the ampys have a type, so Black Baccara being a slice type gets bonus exp from attack Ampys, and normal exp from HP and Def Ampys. Last thing to point out is you can only give 100 of each type per knight.

Now i mentions its cheaper imo, this being my idea, but it does depend on if your going to give all types of ampys or just one type the your knights.
If your going to level the Knight to level 70 and then give ampys then that exp is wasted, Period !
If your not giving all types of ampys, then you might be better off using ampies at lower level and then getting to level 70 with level 100 Manyu.
Do not have that patience myself, BUT that would be the optimum way of doing this, BUT 3 months of playing i have not earned 100 of each type.

Now if you know your going to give ampys to the knight then it way slower than giving lvl 100 manyu, and you need to give ALOT more to level ie alot more cost, BUT if your going to give them anyway, well it quite simple decision imo.

My two cents although not directly related to "Who to give amyps to?" it might help with how to use them.

Sounds good, currently farming secret garden quests for manyu and bought tons of ampies from the christmas event. So if Slice gets xp bonus from attack ampy, what about hit / magic / pierce?

Guessing Pierce will get bonus from Defense (from previous comments). Hit and Magic get bonus xp from HP ampy?

HP ampy gives bonus xp to hit
ATK ampy gives bonus xp to slice
DEF ampy gives bonus xp to pierce

Only magic has no equivalent