You can get one of the 4 star servants below for free during the campaign!
If you don't have a main attacker servant, Heracles or Ibaraki Douji might be the best choice.
Saber
Archer
Lancer
Rider
Caster
Assassin
Berserker
Extra (Avenger)
Note:
Rashoumon event (Ibaraki Douji event)
Murder at the Kogetsukan
Inheritance of Glory
EMIYA
Cursed Arm Hassan
1st Singularity: Orleans
Mysterious Heroine X (Assassin)
Medb
Sakata Kintoki (Berserker)
Ereshkigal
Arthur Pendragon(Prototype)
Gorgon
Black Holy Grail (The Black Grail)