List of Strong Servants in Fate/Zero and Fate stay night/UBW

No.5  Emiya  (Archer-Class,Fate Stay Night)
Archer is a bona fide badass. Because he is not a regular heroic spirit, he lack an actual Noble Phantasm in the truest sense of the team. But his Projection Ability gives him an edge of servants. He has got Rho Aias , a shield capable of stopping even Lancer's Gae Blog , and can mould swords into arrows and shoot them from a mile away . He can summon weapon from the air and fire them just as Gilgamesh , making Archer the only other servant who can neutralize Gil's Gate Of Babylon so effectively. Though each copy is weaker than the original. And then there is a thing called UBW . Archer's versatility and UBW are enough to at least give him a spot in the top 5 .

No.4 Medusa ( Rider-Class, FATE STAY NIGHT)
Rider's  strongest Noble Phantasm Bellerophon allows her to use a devastatingly power charged with Pegasus(similar to Saber's Excalibur).Her eyes turn people to stone . Still between these two abilities , Rider can withstand some of the stronger Noble Phantasm from Archer's UBW and Alexander's Ionian Hetairoi . However she is not a good close-range fighter , making a poor match-up for the other servants .

No .3 Heracles ( Berserker-Class, FATE STAY NIGHT)
Berserker's raw strength is enough to get him into the top 3 . In all seriousness Berserker is the best close-range fighter of all the servants for several reasons. He is the most agile servant in stay night ,making it extremely difficult to corner him. His strongest Noble Phantasm God's Hand makes it practically impossible to actually land a powerful hit on him . Only the strongest of Noble Phantasms can break through it . So he doesn't need to worry about defense at all. Twelve Lives  grants him twelve live and protection from attacks B rank or lower. Any time he is killed by an A rank attack or above, he is revived and granted immunity from that attack in the future. He must be killed twelve times in order to be fully defeated. These abililties made him seemingly invincible .Of course he is not . As Gilgamesh and Saber can and have defeated him one on one . Even so , Heracles raw power is unmatched .

No.2 Arturia Pendragon (Saber-class, Fate/Zero&Fate Stay Night)

She possesses the Noble Phantasm Excalibur: Sword of Promised Victory , a celestial sword constructed by the world from the collective wishes of mankind and fueled through a conversion of Mana into photoelectric energy, allowing a wave of destructive energy to be released from the tip when swung. Another Noble Phantasm she possesses is Invisible Air: Barrier of the Wind King , which compresses and gyrates the air around Saber's blade, turning the blade invisible, making it hard for opponents to judge her strikes, and allowing a one-time release of the air in the form of an intense gust.

Her third Noble Phantasm, the scabbard of Excalibur, Avalon: The Everdistant Utopia bequeaths limited immortality through regenerative stimulation, prevents physical deterioration due to aging, and when activated, is capable of protecting the user from virtually any attack.
With these three weapons , She is nearly unbeatable.

No.1 Gilgamesh(Archer-class, Fate/Zero&Fate Stay Night)

Gil could single-handedly won both Grail War in a day. He has no interest in the Grail and so only participate in it for his own amusement . There is no servant stands a chance against Gilgamesh going all out ,not even Saber . He fought Alexander and Heracles seriously , and absolutely annihilate them . To call them battles is very generous . Gate of Babylon is nearly unbeatable . Only Emiya can match the barrage of swords Gil throw at you and he is ten times stronger .With Enkidu, the chain of heaven he can win almost every battle without even having to lift a finger . He also have the Ea ,the sword of rupture.It is literally capable of destroying entire world ,and even distort the fabric of universe .He is always reluctant to use it because there are very few people he actually  deems worthy to gaze upon its glory. But when he does crack it out ,there's usually a wrap . Gilgamesh is undoubtedly the strongest one in the fate franchise . If he take the fight seriously ,Gil can pretty much demolish every other player in the game.