Any of the 5 stars currently avaliable will get you very far into the game, and all of them can be used in some way shape or form. The rate for 5 stars is 1%, please keep this in mind when rerolling / spending money on this game.
EN Tier list
Note that only 6 of ★5 Servants are available on the EN version of the game.
Waver is an amazing support Servant who will be useful even into the later game, as he can provide the most NP charge to other members of the party, in addition to good buffs on attack, defense and crit damage. His own NP has to ability to stall by stunning and decrease the enemy's NP charge while also inflicting a defense down. Keep in mind that he is a support servant, and will do just that. He won't be dealing a huge amount of damage.
NP deals extra damage against most servants, and he also has an NP gain up skill. His NP gain is further enhanced by his ability to use stars (star absorb skill), and the standard Independant Action allows him to deal huge amonts of crit damage. Keep in mind that he is only avaliable on a limited Gacha, so he isn't the best reroll target.
Jeanne is an amazing servant in the early to mid game, as she resists damage from all classes except Berserker. She also provides party-wide invincibility for a turn, which can be used to absorb the damage from enemy NPs when the battle goes long. In the later game she starts to fall off because her NP stuns her for two turn and she has low damage potential, combined with the introduction of enemies with invincibility piercing abilities. However when she recieves her interlude, the stun demerit is removed from her NP, which makes her useful once again.
At launch he likely one of the strongest servants in the game, due to being the only 5 star Berserker (with stats to match). However be will truly shine when Tamamo no Mae is released, who will allow him to repeatedly use his NP to great effect. For now, he can do well just from his stats and class.
The Flagship Saber, can provide benefits to the team with her Charisma and has Prana Burst to enhance her Noble Phantasm. She has high HP and her NP has an overcharge effect of gaining NP charge. However as it is not a single target NP it will not burst down bosses quickly.
Altera is a powerful Saber class servant, with selfish skills that enable her as an attacker. Her NP has the capability to obliterate waves, especially when paired with her buffs. An important thing to note is her final skill unlocks with her third ascension which is currently hard to obtain. With the highest attack stat in the current state of the game, and her divinity passive she can and will obliterate targets, even more so with a brave chain and/or crits.
Regarded by most as the most broken utility servant of all time as he brings everything to the table. Healing, NP charge, even party wide Invulnrability and thats without going into his NP (Which gives Stars / Healing / NP charge per turn).
Before Merlin was released, Waver was considered the best support servant, right now he fills a different role. He shines at building NP gauges, being able to provide 50% to a single servant on turn 1, while also buffing their Attack,defense, critical.
Jeanne Alter brings damage in spades due to her class and attack stat. She gains NP very fast when being hit (which can be combined with her invulrability) and has decent NP gain when attacking. Her skills compliment her well and also allow her to effectively use stars on some turns.
He has the strongest self buff in the game, which comes with a downside of only 60% activation chance. However Ramesses has an NP gain skill that also buffs buff activation chance. His star generation is decent, but not the best in the rider category. His NP is a powerful single target Buster which can inflict NP seal.
He has high attack in combination with being a Berserker, he does a large amount of damage even on his own. His skills are defensive orientated, which helps him survive longer than a Berserker should. He can generate NP well, and his NP itself buffs him before the attack, resulting in a very powerful attack.
Can absorb stars, and has a lot of self-buffs which enable her to deal damage. However, her star absorb is most effective when used in combination with her Arts card to build her gauge. Yorimitsu’s NP does not do much damage but it can generate an amazing number of stars, which she can then make use of in the next turn. Other than her NP she doesn’t have great star generation though so she is reliant on others.
NP deals extra damage against most servants, and he also has an NP gain up skill. His NP gain is further enhanced by his ability to use stars (star absorb skill), and the standard independant action allows him to deal huge amonts of crit damage.
Artoria's NP is a single target Arts, which can be combined with her skills to cycle her NP very effectively. Her cards provide good NP gain and can be used with some servants to NP twice in a row. Her skills bring both self support and team support, and giving her decent sustainability
Good team buffs, brings survivability to the team in the form of guts and she can draw stars to herself, however she doesn't generate many stars for herself. While her NP damage is standard, she can buff herself up before using it.
A star generation monster due to having a huge amount of hits on her quicks. Her NP also deals a lot damage while ignoring defense, making her very consistant (it also does 50% extra damage to Female targets). Note that she doesn't make the best use of her own stars.
Standard Buster AOE NP, but is supported by very good self buffs with low cooldown and an NP charge skill. Her skills can also provide Incvulrability and pierce.She can also do good damage and generate NP with her cards.
The NP spam enabler, lowers team skill cooldowns, and charges their NPs. WIth her signature Arts buff she can push other servants damage and NP cycling. Note that she is very support orientated, and has difficulty on her own.