It depends of what kind of servants you need more. Though, you can always have count on Morgan to do majority of things.

If we're only talking about gameplay, Ciel is ridiculously strong and versatile for the most difficult farming.

Ah yes, judging servants only based at their farming ability.

I see Draco, I vote Draco.

If we are looking at rarity than it has to be Draco.

If you already have Merlin\LA and Castoria - than it's easy Morgan, if not - than you should get Merlin and Castoria.