Fate/Stay Night lives and breaths on its Servants. The interpretations that Type-Moon puts on the myths and legends that participate in the Holy Grail War are an important and intriguing part of the series. Though, while some are stronger than others, some make for a better Servant for a master in the Grail War.
Even the strongest Servants can have a horrible drawback that puts them out of the race for best Servant, while some have other abilities that qualify them to be one of the best. The best and the worst isn't simply a measure of who is stronger.
10 Best: Caster
Among the original Servants that appear in Fate/Stay Night, Caster is considered to be one of the weakest. This is true because in most one-on-one confrontations she is wholely outclassed by most Servants. Even Rin beat her up one time. Thankfully she has a few abilities that help her skimp by into the "best of" territory.
First of all, Rule Breaker! Actually, make that second of all, third of all, and last of all. Rule Breaker is an excellent Noble Phantasm that allows the Servant to do just that, Break Rules. It allows her to manipulate the Servant System by severing Servant contracts and binding them to herself. Then you have the fact that she can summon a Servant of her own...
9 Worst: Assassin
Assassin's True name is Sasaki Koujiro, a famous swordsman said to have faced Musashi Miyamoto in battle. Except that is not actually the case. What was summoned in place of Sasaki Kojiro was someone who fits his parameters and not the figure in question.
Despite not being the actual swordsman, he does bear most of his techniques. Unfortunately, he has one huge detriment: He is stuck to one place during the entire war. Due to how he was summoned, he haunts Ryuudou Temple as its guardian. Forcing anyone who summons him to hole up there with him. Caster uses him to great effect but most masters would not be able to do the same.
8 Best: Rider
Is she the strongest? Nope. Is she the most resilient? Not by a long shot. Does she have the best Noble Phantasm? She is up there but still not quite the best. So why does Rider land on this list as one of the "best" servants? Because while outclassed by other Servants, no other Servant matches her loyalty in the Holy Grail War.
It isn't revealed until the Heaven's Feel route of Fate/Stay Night, but Rider's true Master was Sakura Matou. Shinji Matou was only there as a stand-in because Sakura does not want to fight. Rider then dedicates every action during the Holy Grail War to protect Sakura. Sacrificing her pride, her honor, and even herself to help the one she holds dear. What more can you ask from a Servant?
7 Worst: Avenger
It can be forgiven if this is the first you have heard about this Servant. Only appearing in Fate/Hollow Ataraxia, it is a visual novel considered more of a side story to Fate/Stay Night than a proper part.
The reason he is considered the worse is that he is the weakest Servant of the bunch with a lousy Noble Phantasm to boot! Verg Avesta: Falsely Transcribed Creation sounds good on paper because it reflects all damage suffered by Avenger back to his opponent's soul. The only problem is that he has to remain alive and conscious for it to have any effect, as it ceases to function if he is unconscious or dead. Worse yet, he has very few redeeming personality traits.
6 Best: Archer
Provided that Shirou doesn't participate in the Holy Grail War to instigate Archer's attempt at assisted suicide, Archer is an excellent Servant all around. He may be a bit of a sourpuss but Archer is relatively easy to get along with. As long as you don't immediately get off the wrong foot with Archer, he will be a devoted protector and guardian during the Grail War. He is also a wonderful homemaker and chef!
Though what puts him on the map is his ability in Projection Magecraft. It allows him to adapt his style to whatever Servant he is currently facing through the use of copying varied Noble Phantasms and other weapons.
5 Worst: True Assassin
This Servant was good enough to make it onto a list of 10 Strongest Servant in the Fifth Holy Grail War, but that doesn't make him the best Servant for a typical Master. True Assassin has a plethora of special abilities that make him a dangerous Servant but his true power comes from subterfuge and assassination rather than direct combat.
So the Master has to be able to utilize his skillset well to be successful in a Grail War. He has to never let Assassin engage other Servants in a fair fight, aim to kill their master instead, and overall play a constantly dangerous cat and mouse game with every other Master in the War. Something quite outside the purview of any old Master.
4 Best: Lancer
Cú Chulainn is basically Irish Hercules. His story as detailed and legendary as the great hero of Greek myth and coming with his own set of incredible powers that would mean trouble for any opponent he faces. Including one that alters casualty to strike an opponent's heart.
Furthermore, he possesses several personality traits that'll make him easy to get along with. He has a carefree but playful attitude that is only tossed aside when engaged in battle. He's the type that would never break a promise and would loyally guard his Master through the entirety of a Grail War.
3 Worst: Berserker
Here is the real Hercules. The big brute possesses the insane skills and strength befitting of the hero of legend. A force that few can contend with in any matter of circumstances, let alone a straight one-on-one fight like most battles in the Holy Grail War. So why is he a terrible Servant for a typical Master? Two words: Mana consumption.
Hercules is like an open faucet of mana consumption set to full blast. It is a struggle to merely maintain his existence in the world, let alone keeping up the requisite amount of magical energy needed to fuel him in battle. A Master is most likely to end up a husk devoid of magical energy if they are linked to Berserker in a Holy Grail War. Only people specifically built to maintain such power like Illya can prove to be a good fit.
2 Best: Saber
Saber is the epitome of an excellent servant in all ways, shapes, and forms. It is no wonder that the Saber Class is widely considered to be the best and most likely winner of a Holy Grail War.
King Arthur is a master swords(wo)man equipped with the most famous sword in history. Her skills with the blade complement Excalibur's pedigree as the Sword of Promised Victory. Her magic resistance is strong and her defense only gets better if you manage to get ahold of Excalibur's scabbard, Avalon. Better yet, as long as you aren't a scumbag she is easy to get along with.
1 Worst: Gilgamesh
Gilgamesh is by far the strongest Servant in the 5th Holy Grail War. He has a treasure trove of Noble Phantasm he can throw at his opponent, including a weapon that can be considered an anti-world Noble Phantasm if Gilgamesh decides to unleash its full power. By all regards, he should be the Servant to aim for if you want to win the Holy Grail War.
Except that Gilgamesh is incredibly hard to get along with. It is very easy to set him off and any perceived slight could mean the end of life for any Master in charge of him. He is also more than willing to toss you aside if he finds you useless. He did it to Tokiomi Tohsaka, he did it to Shinji, and he'll do it to you.