Neither said: Judging on how overpowered the other servants are, I think not.
Depending on the application of her abilities, and ruthlessness of her master, Jack is probably the most deadly. She has absolute stealth and a guaranteed first strike. She also has an attack that can be instantly fatal to any female, and do devastating damage to others who have poor curse resistance. Simple answer, kill all the other masters and disappear into the night. Also flee from any direct confrontation.