This is purely opinion based, What legendary heroes do think that if they were made servants that by through myth, fame or history would game-breaking in a Holy Grail ,like for example take Gilgamesh the literal definition of broken as a Servant. He would be the most obvious example but others would include those in the reference. So what heroes do you think fit the bill(And by heroes I'm talking those that haven't been made as a Servant by Type Moon yet.)
Who do you think can be the greatest heroes..
Reference:
Canon Servants who can do this.
Spoiler:
Servants that should be able to do this but either it isn't shown clearly or they are represented weakerSpoiler:
(lists are subject to change)