Caster is overpowered if she has enough mana, but I don't see her killing people for it nor even making them sleep. She would need an erotic black hole of a master, if you know what I mean. Wonder how many servants would be befriended.
Saber would be the strongest in terms of his legend, because everyone heard about him and how dangerous he was. But he would also be wary of killing anyone. In techs, he would be like a much stronger Fake Assassin.
Berserker would never acknowledge a master unless one of the other masters or servants was a member of the God Hand, lol.
Rider wouldn't last long in a serious fight. Can't think of any feats to measure her power...
Archer depends on the Caster Bullets he can use, and if his ship is available in some form.