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Firstly, if you don't know anything about the mechanics, read them http://kairosity.tumblr.com/post/125576872324/fategrand-order-how-to-play here.
Also, you don't need to reroll any of these. And if you like the servant, they'll all totally usable. The game isn't remotely hard until Camelot, which is months away, and even then it's beatable with specific silvers and support servants. Frankly, anything above a 3* is just a bonus in this game. That said, this guide is written with recommendations in case you do want to reroll for something optimized.
Siegfried
Saber
ATK: 8,181 (At Level 80)
HP: 14,165 (At Level 80)
Cardset: Quick/Arts/Arts/Buster/Buster
Skills
(All at Rank 1)
Golden Rule: Increase own NP Gain by 15%. (3 Turns) (Cooldown: 8 turns)
Disengage: Remove all debuffs from self, heal self for 1000 HP. (Cooldown: 7 turns)
Dragon Killer: Increase damage against dragons by 50%, reduce damage from dragons by 30%.
Noble Phantasm:
Balmung
Buster
Powerful attack (300%) to all enemies
Bonus damage (150%) against [Dragon]
PROS
Absolutely destroys dragons, which Orleans is filled with. A few lategame bosses are also dragons.
High health.
Will eventually get a buff to both his third skill and his NP, to make them more useful against non-dragons.
Is a Saber, which is a hard class to come by.
CONS
Fairly low attack for a 4*.
NP is low power against non dragons, and third skill is outright useless against nondragons.
A high health AOE Buster 4* Saber, meaning he fills the same role as the free Saber Lily.
Most dragons are riders, so he doesn't have class advantage over his natural foe.
Overall: 5.5/10
Recommendation: Reroll
D'Eon
Saber
ATK: 8,765
HP: 13,625
Cardset: Quick/Arts/Arts/Buster/Buster
Skills:
Mind's Eye: Give self Evasion for one turn, increase own defense by 8% for three turns. (Cooldown: 8 turns)
Self-Suggestion: Remove all debuffs from self, increase Debuff Resistance by 50% for three turns. (Cooldown: 7 turns)
Perfect Beauty: Force all enemies to attack self for three turns, heal self for 1000 HP. (Cooldown: 7 Turns)
Noble Phantasm:
Fleur De Lys
Arts
10% Chance of Inflicting Charm To All Enemies
Reduce attack of all enemies by 10% for 2 turns.
Reduce defense of all enemies by 10% for 2 turns.
Pros
High HP.
Long-lasting taunt.
All skills increase survivability.
NP can be spammed.
Cons
Lowish attack for a 4*.
No offensive skills.
NP does no damage and its debuffs are fairly weak.
Same class as Saber Lily, though her role is different.
Overall: 7.5/10
Recommendation: Personal Preference.
Emiya
Archer
ATK: 9,398
HP: 11,521
Cardset: Quick/Arts/Arts/Arts/Buster
Skills
Mind's Eye: Give self Evasion for one turn, increase own defense by 9% for three turns. (Cooldown: 8 turns)
Clairvoyance: Increase own critical star rate by 16% for three turns. (Cooldown: 8 turns)
Magic: Increase damage of own arts cards by 21% for 1 turn. (Cooldown: 7 turns)
Noble Phantasm:
Unlimited Blade Works
Buster
Powerful attack (300%) to all enemies that ignores defense.
Reduce enemy attack by 10% for one turn.
Pros
Cardset allows for quick NP Generation.
Well-balanced stats.
Fairly survivable due to first skill.
Second and third skills, plus NP, will eventually be buffed.
Cons
Second skill is underwhelming pre-buff.
NP is somewhat weak pre-buff.
Overall: 8.5/10
Recommendation: Keep unless strongly disliked.
Elizabeth
Lancer
ATK: 9,122
HP: 11,870
Cardset: Quick/Quick/Arts/Buster/Buster
Skills
Charisma: Increase attack of all allies by 8% for three turns. (Cooldown: 7 turns)
Torture Techniques: Reduce defense of targeted enemy by 10% for three turns. (Cooldown: 7 turns)
Battle Continuation: Give self a buff that, upon death, will be expended to revive self with 750 HP. Buff lasts for five turns. (Cooldown: 9 Turns)
Noble Phantasm
Bathory Erzsebet
Buster
Powerful attack (300%) to all enemies that ignores defense.
Curse enemies for 3 turns. Cursed enemies will take 500 damage a turn.
Pros
First skill will eventually be buffed to become very strong.
Balanced stats.
Fairly survivable.
Cons
First skill is underwhelming pre-buff.
Fairly weak NP, with almost pointless debuff.
Overall: 7.5/10
Recommendation: Personal Preference
Marie
Rider
ATK: 8,293
HP: 12,348
Cardset: Quick/Quick/Arts/Arts/Buster
Skills:
Alluring Voice: 50% chance to Charm target Male for 1 turn. (Cooldown: 9 turns)
Beautiful Princess: Invulnerability against next 3 attacks to hit self, heal self for 200 HP each turn for 5 turns. (Cooldown: 8 turns)
Grace of God: Increase own chance of landing status effects by 10% for 3 turns, heal self for 1000 HP. (Cooldown: 7 turns)
Noble Phantasm
Guillotine Breaker
Quick
Powerful attack (600%) to all enemies
Heal all allies for 500 HP
Dispels debuffs of all allies
Pros
High HP.
Very survivable.
Very good second skill.
Cons
Third skill is just a slight buff to first skill, essentially making it a wasted skill slot.
No offensive skills.
Low attack.
Weak NP
Several decent bronze and silver riders.
Overall: 5.5/10
Recommendation: Reroll.
Martha
Rider
ATK: 8,014
HP: 13,068
Cardset: Quick/Arts/Arts/Arts/Buster
Skills:
Protection of the Faith: Increase own debuff resistance by 50% for three turns, heal self for 1000 HP. (Cooldown: 7 turns)
Miracle D: Remove debuffs from all allies, and heal them for 500 HP. (Cooldown: 7 turns)
Noble Phantasm
Tarasque
Buster
Powerful attack (300%) to all enemies
Reduce defense of all enemies by 20%
Pros
High HP
Strong support skills
Support servants are hard to come by
Great debuff on NP
Cons
No third skill until her strengthening quest is unlocked
Low attack
There do exist two or three decent bronze support servants
Overall: 7/10
Recommendation Personal Preference
Stheno
Assassin
ATK: 8,985
HP 11,518
Cardset: Quick/Quick/Quick/Arts/Buster
Skills:
Bloodsucker: 60% chance to decrease target enemy's NP Gauge, increase own NP gauge by 18%. (Cooldown: 8 Turns)
Alluring Voice: 70% chance to charm target Male enemy for 1 turn. (Cooldown: 7 Turns)
Noble Phantasm
Smile of the Stheno
Arts
100% Chance to Insta-Kill Target Male Enemy
100% Chance to Charm Target Male Enemy
Reduce Target Enemy's defense by 20% for 3 turns.
Pros
Can Charm males very effectively.
NP is strong against males.
Decent critical star and NP gauge generation.
Very useful against male bosses.
Cons
NP is near useless against Female or genderless enemies, and deals no damage.
No third skill until strengthening quest unlocks.
Second skill is also useless against Female or genderless enemies.
Overall: 7/10
Recommendation: Personal Preference.
Carmilla
Assassin
ATK: 9,408
HP:: 10,473
Cardset: Quick/Quick/Quick/Arts/Buster
Skills:
Bloodsucker: 60% chance to decrease target enemy's NP Gauge, increase own NP gauge by 18%. (Cooldown: 8 Turns)
Torture Techniques: Reduce defense of targeted enemy by 10% for three turns. (Cooldown: 7 turns)
Noble Phantasm
Phantom Maiden
Buster
Extremely powerful attack (600%) to a single enemy
Bonus damage against Female (120%)
Heals self for 2000 HP
Increase own attack by 20% for 3 turns
Pros
Fairly high attack.
Decent star/NP Gen.
Incredibly good Noble Phantasm.
Decent debuffs.
Cons
No third skill until strengthening quest.
Overall: 8.5/10
Recommendation: Keep.
Tamamo Cat
Berserker
ATK: 9,026
HP: 11,458
Cardset: Quick/Arts/Buster/Buster/Buster
Skills
Monstrous Strength: Increase own attack by 10% for 2 turns. (Cooldown: 7 Turns)
Witchcraft: 40% chance to reduce target enemy's NP Gauge. (Cooldown: 7 turns)
Transformation: Increase own defense by 18% for 3 turns. (Cooldown: 7 turns)
Noble Phantasm
Sansan Nikkou Hiruyasumi Shuchikurin
Quick
Powerful attack (600%) to all enemies
Heal self for 2000 HP each turn for 3 turns.
Stun self for 2 turns.
Pros
Berserker, so useful in most battles.
AOE NP and class make her good for farming.
Survivable due to NP's self heal.
Balanced stats.
Cons
Berserker, so needs to be supported even with her heal and defense buff.
Self-stun on NP.
Fairly weak second skill.
Overall: 8/10
Recommendation: Keep.
Heracles
Berserker
ATK: 10,655
HP: 10,327
Cardset: Quick/Arts/Buster/Buster/Buster
Skills:
Bravery: Increase own attack by 10% for three turns, increase own debuff resistance by 21% for three turns. (Cooldown: 7 turns)
Mind's Eye: Evade all attacks for 1 turn, increase own critical damage by 18% for three turns. (Cooldown: 8 turns)
Battle Continuation: Give self a buff that, upon death, will be expended to revive self with 1000 HP. Buff lasts for five turns. (Cooldown: 9 Turns)
Noble Phantasm
Nine Lives
Buster
Extremely powerful (600%) attack to a single enemy
Reduce target's defense by 10% for three turns.
Pros
Berserker, so useful in most battles.
Hits like a tank due to cardset, NP, Bravery, stats, and class.
Skills add survivability.
Extremely powerful NP.
Cons
Somewhat low HP.
Frail without two defense skills up.
Overall: 9/10
Recommendation: If you're rerolling for the best servant, this is the one you want.