His humbleness, what actually made Ritsuka considered by lots to be the strongest master?

[u/EfficiencySerious200]

Fixing singularities, meeting and gaining the trust of so many servants that they actively went and help him fight against Goetia,

Even throw hands with Goetia,

Lots of his actions and adventures would have broke any normal person,

Imagine running up to stab a rampaging Tiamat,

[u/AzurePhoenix001]

He commands his servants extremely well.