Fate/Grand Order boasts an enormous roster of servants that continues to grow with each event and update. As of now, there're almost 130 4 star servants in the North America version of the game. Having so many different servants is what makes Fate/Grand Order so special in the eyes of its players.
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Each servant can support a team in specific ways . This could mean specializing a team to focus on a certain kind of attack, buffing allies, dishing out huge damage, etc. What determines the tier of a servant is how versatile they are, how powerful their abilities are, and if they're useful in event battles since those give the best rewards.

S Tier

S Tier servants are capable of matching up with some of the best 5 star servants in terms of capability in battle. Teaming up the right 4 star with other servants could spell the difference between victory and defeat. Mash and Heracles are two great examples that are available from the very beginning of the game. RELATED: Fate/Grand Order: How To Reroll
Mash is given to the player by the story and as it progresses, she becomes one of the most useful units in the entire game . Heracles is a featured unit in the tutorial gacha so he is also available from early on if the player chooses to reroll until getting him.

A Tier

For A tier, specialization is key. These servants are often the strongest in their class and category for 4 stars. 4 star servants also require a bit more team support to work well so their ability to work well with other units is a big factor in determining how good they are.
New players should look out for Emiya since he is also available in the tutorial gacha. Having Emiya and Heracles is a great start to a free to play account and can be achieved with a few reroll attempts. Emiya's skills are great for working with a team but his deck's strange structure can make team optimization difficult.

B Tier

B tier servants make for good all around teams, that steam through low priority enemies and levels. Some have uses for more difficult fights, but mostly need help to get the job done. These servants are great overall, but can't hold a candle to those in the tiers above. RELATED: Fate/Grand Order The Complete 3 Star Servant Tier List
Martha and Elisabeth Bathory are the servants available in B tier that appear in the tutorial summon. Martha is one of the few servants capable of removing all debuffs from party members, but her damage is severely lacking. Elisabeth has a similar situation since she can buff female party members with up to 60% extra attack power. For a starter-niche character, she is fun to use but doesn't hold up during later missions.

C Tier

These servants can have uses in certain teams but are heavily outmatched by the higher ranked characters . If there's no alternative, these characters can have a time to shine, but they must be supported by the team structure well.
The units worth mentioning of this tier are:
-Saint Martha (Ruler) who is a very specialized character. Her cards are mostly buster, so she can grant attack buffs. She also has a defense debuff that's useful for taking down tougher enemies. Ruler class servants are pretty rare so it's unfortunate that she isn't more useful, but she has her moments.
-Archer Of Inferno is another solid buster specialist who is great for single boss fights. Her kit and ability to counter certain gimmicks make her a solid investment as long as she isn't used in a primary team. Her lack of AoE attacks makes it difficult to choose her over other 4 star archers who can target multiple enemies at once.
-Marie Antoinette is a rider unit built unlike most riders in the game. Instead of being focused on aggression, Marie Antoinette is designed to survive. She has a great invincibility skill and two ways to restore HP. This coupled with her high HP value make her very useful for drawn out battles with many waves of enemies. Her biggest downside is her offense, as she struggles to damage on her own.

D Tier

In D tier, Thomas Edison is a servant with lots of potential. His stalling capabilities make it easy to build up Noble Phantasms if the team is centered around arts. His setup is unique and that's what makes him fun to play. As the game evolves, he might have more use than he does right now so keep an eye out for that.
The issues with Thomas Edison are the same issues that a lot of D tier servants face. Their kits are interesting but just don't flow well or aren't that useful when other strategies are so much more powerful.

E Tier

These servants can be used and grant good results but it takes a lot of dedication . Many of their skills and kits don't make much sense to use since others have it way better. For example, Yu Mei-ren is an assassin with an AoE Noble Phantasm. On paper that's attractive and useful but once she's put in practice, she falls apart. Her Noble Phantasm is unique and removes debuffs as well as buffs from her before giving her a buster buff. The fact that she can't be buffed before using the ability kills its viability and compatibility with her team-mates.
Overall, E tier servants are still viable so don't take their placement as a message to not use them. If they're interesting to the player, they should be used if there's a way to make them work. Tier lists are not cut and dry in terms of who is more fun to use, they just indicate what servants can win efficiently given the current state of the game. NEXT: Fate/Grand Order Developer Experiences Massive Drops In Profits