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.
Currently, Kintoki's NP Brave chains are able to deplete a significant portion of most enemies' HP, making him a highly effective Servant. His performance, however, will continually decrease as opponents' HP amounts are increased later on in the game.
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.
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.
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.
Having most of the kit needed for hard quests (damage buffs, healing, star generation, party Invulnerability, and NP charge), Merlin allows players to clear challenges way more easily. His notable weaknesses are that he is unable to provide a defense buff, and that his Invulnerability skill has a rather long cooldown.
Providing a significant on-demand NP charge as well as performance boost for a certain duration, Zhuge Liang is most suited for fast-paced battles. As he lacks healing and significant defensive skills, however, he is somewhat unsuited for drawn-out fights.
With her NP's stun no longer in the picture, Jeanne is now a highly consistent defensive Servant, providing a teamwide Invulnerability, healing, and the coveted party debuff removal. These factors place her as one of the most sought after Servants for harder content.
Possessing a very high ATK stat and the most damage-focused skillset among virtually all ingame Servants, Jeanne Alter's offensive capabilities in a critical team is unmatched, though due to her Avenger class, she will deal neutral damage most of the time.
While Ereshkigal possesses notable support capabilities, these are not exactly what we would call 'decisive', especially given Protection from the Underworld's only 50% uptime. While she can also be used offensively, her lower NP damage and lack of critical absorption leaves her in an awkward spot, where her QQABB deck compromises her mainly supportive role.
How come Scathach is on S tier while Tamamo Lancer is on A tier.
Tamamo requires way less set up due to better Np gain and having a buster NP which allows her to be pair with other Buster support like Merlin, Leonidas, Shakespear, Nightingale etc.
Scathach despite having better NP damage and effect. Suffer from bad Quick cards despite having a Quick NP (which make it's really difficult to find a ce that fully synergized with her) and lack of dedicate Quick support in the current meta.
If I am not mistaken, the reference of these tier list was taken from Gamewith.jp one, instead of the Appmedia one. Appmedia is somewhat became notorious for ridiculous tiering that triggered not only the JP players, but also the overseas playerbase as well.
While Vlad does not have a lot of damage, he IS a ST berserker. NA is still in early game, which means a good NP or two will kill a boss, pretty much. He also synergizes well with Mashu and either a healer or Waver, so it's actually not too surprising that he can be ranked so high in NA. Vlad does get powercreeped in later chapters, but with Tamamo on the way he won't ever fall too low.
I believe they rate him highly due to the fact that he's the only non-limited 5 star berserker at the current time. This plus his future potential makes him pretty solid. Maybe they got lazy and don't want to keep updating his rating.
What about Okita Souji? isnt she supposed to be on par with Gil?