Lorewise, who is the strongest servant here?

Beast Class Draco.

Arthur Pendragon with Avalon, a fully unsealed Excalibur, and a living body so he can make sure of the dragon core inside him that grants him nigh infinite mana.

100% power Arcueid fighting under a full moon with the full backing of Gaia and the Counter Force

Kuku at the peak of her power as seen in Lostbelt 7

Lostbelt Morgan sitting on the throne with hundreds of clones already summoned and her divine spears ready to be deployed

Beast Class Kiara at the peak of her power in the Mooncell

Space Ishtar from the servantverse at the peak of her power

The Lostbelt version of Arjuna that absorbed all of the indian gods

Indra at his full power which is apparently so great that him simply manifesting shakes the skies

Serious Gilgamesh as well, while assuming he's wearing his armor, his clairvoyance, his battle precognition, and let's also assume he's dual wielding EA and Enkidu and riding atop his faster fhan light spaceship

Ordeal Call Paladin Class Mash who has surpassed Galahad

Or Void Shiki with the mystic eyes of death perception and access to the Root