Gilgamesh

The Archer of the previous Grail War. Mankind's oldest king of heroes, who defeated Saber. For more details, refer to his status screen in the game.
As no proper Heroic Spirit can match this man, he could be called the “Servant Killer.”
Without a doubt, he is the strongest existence amongst the Servants.
Though he lost due to obsession against Saber…
Though he lost due to pride against Shirou...
Though he lost due to carelessness against XXXXX…
…The fact of the matter is that if he actually gets serious, he is a Servant that cannot be rivaled.
Perhaps because he has been living in human society for the past ten years, he seems to be hip on the latest fashions. He owns various casual clothes, but his favorite is the biker outfit he wore in the Rin route.
Well, he liked it enough to overlook Shirou's group and leave instead of letting it get dirty, at any rate. I can't believe the protagonist's life was worth less to Gil than his coat…!
It might seem strange how Gil could live in human society for ten years with a personality like his and not cause a fuss, but Gil must have realized it would have been a problem, too, because apparently he drank a certain Noble Phantasm to change his appearance and personality.
Also, during the design stage, I selfishly insisted that “Gil absolutely has to wear full golden plate armor.”
Why, you ask? Well, it might be because I was repeatedly challenging a certain 60 story tower at the time…

Fate/side material: Fate Encyclopedia
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The Archer class Servant of the Fourth Holy Grail War. Though he was summoned by Tohsaka Tokiomi, he ended up deserting his master and re-contracting with Kotomine Kirei. He is the strongest character in both Fate/stay night and Fate/Zero.
In the boy-meets-girl story of the original Fate/stay night he was forced to yield to the universal law of “love conquers all,” but in Fate/Zero he's free to show off his overwhelming, cheat-like strength without restraint. He could have ended the war in a single night if he truly desired to do so. But then, careless pride is the king's prerogative, and getting serious over something as frivolous as a wish-granter would be unbecoming. So, taking that into account, it sort of balances out the huge disparity in firepower… I guess?
Also, the reason that he was able to splendidly escape after being swallowed by Angra Mainyu at the end of Fate/Zero despite succumbing to the same thing in the Sakura route of Fate/stay night is because in the latter he was distracted by the presence of a naked girl.
…Just kidding! Actually, it was due to the difference in power between the “mud,” which was cut off from its source, and the “shadow” which was connected directly to it.

Fate/Zero material: Fate/Zero Encylopedia