Mashu is the Servant that stays with you from start to finish (mostly) and she boasts way more advantages than disadvantages. She excels at defensive buffs with her skills and NP, which can also protect her teammates while almost constantly providing defensive buffs and filling up her NP gauge to unleash the cornerstone of her defensive powers. Once her true NP is unlocked in Camelot, she can also support offensively. Early in the game (and NA version), she may take a very slow pace to unlock her true potential, but even with that, she's still providing a lot for the team, so when the time comes, the already good Servant just became... better.
Andersen is THE ultimate Low Star Support Servant when it comes providing buffs. He has most of the buffs needed for any team compositions and he has a tremendously excellent NP regeneration with both his default NP Regeneration point and his 2nd Skill, at maximum level, will fill his NP gauge for 75% in 6 turns, which mitigates his flaw of shaky NP buff chance. He's fragile and weak, but it's easily covered with Command Spells or other Servants, and he does not have surefire protection buffs (Evade or Invincibility), but even without it, the list of supports he provided are already versatile enough. If you do not have Zhuge Liang or he costs too much for your team, Andersen can easily fit in the bill.
Cu Chulainn is known the 'ultimate tank' not for nothing: While his offensive spread is lower than average, he makes it up with extreme survivability that can end up having him solo many of the most difficult opponents solo and come out victorious with steady damage. Unlike most Evade/Invincible skills, Protection From Arrows doesn't expire if the enemy does not hit Cu Chulainn, so he can keep his invincibility while also setting up his Guts to come back after death. Unfortunately he's not quite the team player and doesn't share most of his benefits to his allies, but a team built on him is possible and can be devastating.
David is a tremendous survival Support. Not a lot can induce a party-wide Evasion, healing and removing debuffs in the same time, and it also heals with respectable amount. If you are running low on mass healers or your Master Skill is down, David is the way to go to keep your party alive in prolonged battle that contains AOE damage enemies. And perhaps most importantly, he's a reward Servant for clearing the Okeanos chapter and raising his damage-dealing NP isn't his priority as a Support, so you don't need to throw a lot of Saint Quartzes to get the most of him.
Amongst budget Servants that hail in 1-Star category, Arash is known as the best, because a huge damage from his NP, Stella, is something that can compare with other high-rated Servants in sudden damage. Think of him like a ticking time bomb, the longer he charges, the greater damage he causes, but afterwards he's gone, unless you put Guts on him via CE or other Servants (Irisviel). Like other 1 Star Servants, his other areas would become lacking, but his NP also causes him to be one of the Servants where you don't have to worry about stats and damages, popping in a nuke in form of Stella (while dying) is his purpose, he's the best there is at that field and if he has a good team behind him, he'll be able to produce a great damage output faster (he can charge his NP alone, but it's not as big of a charge as some Casters).
As a low cost Tank, Leonidas will serve well for a team looking for one. However, despite having two tanking skills, Leonidas is trickier to use due to how he only has 1 Art Card and his main Taunt is his Noble Phantasm, Thermopylae Enomotia, which means he will have a hard time to reach his main tanking option, so he'll also need help from NP generation from other Servants. Unlike St. George, however, he sacrificed some defensive capabilities with other utilities like Crit Star addition with his NP. His third skill (still locked in NA) also allows him to support Buster Teams. Leonidas would fit greatly if the enemy has many hit counts (e.g: Gilgamesh) since his secondary taunt increases his own NP regeneration, but overall, he's a bit trickier to unleash his true potential.
George is a pure team tank character, so if your team needs tank that protects others (instead of being a survivor) and nothing more, he's your man. Unfortunately, that's kind of all he does, he's so specialized at protecting your team that he offered almost nothing else, except of rare cases that you have an urge to use Siegfried, in which his Noble Phantasm work well with Balmung. Outside that, he's rather lacking in other areas, but he provides a steadier protection with his self-heal and 3-turn Taunt as a skill (as opposed to Leonidas), so you may want to call him if your team just wants a pure protector tank.
A critstriker is someone who consistently does critical strikes a lot of times, and that's what Kojirou does best, aside of slaying dragons and saving players from them in Orleans. With proper support, Kojirou can increase the power of his criticals to a surprising degree and he comes with a slight tanking option with his Mind's Eye (Fake) to take out single targets quickly. Unfortunately, for a damage dealer, it's kind of hampering Kojirou's damage potentials that he's a Quick-based 1-Star Assassin, but at the very least it will help him fulfill his role as a critstriker and helps him unlock his full potential quicker.
Caesar is the shining example of 'Never judge a book by its cover'. Despite being presented as a lazy fat man that's a far cry of other Caesar depictions, he is one of the best budget Sabers with good all around stats, ability to boost the offense of himself and his team's and a devastating single target Noble Phantasm, also quick based, enabling the party to do criticals more often. His survivability is rather low, but he will definitely serve you as a damage-dealing Saber you can trust until you somehow roll a higher rarity Saber.
Medusa is decent, to say the least. She deals decent damage amongst the 3-Stars, she has a lot of manners to fill up her NP. The problem is that most of the methods are hampered with low hit counts in her animation, and the third skill, locked within a Strengthening quest, didn't improve her NP gain much. That said, a 100% Stun for 6 turns with her first skill is something that can be a lifesaver when you're fighting one boss and trying to stall time, and she can at least spam a level 1 Noble Phantasm in order to clean the enemy party more constantly while improving a Critical-gathering team. She definitely needs CEs to help optimize her skills.
Now let's look aside about how Teach is a (hilarious) creep who lusts after young girls and indulges in lolspeaks, he's actually a pretty well-rounded 2-Star Servant that has clear ways about how he's built. He supports his team's offense well with his mass attack/NP power boost and he can buff himself further to go on the offensive himself, although his NP's side effect are very lacking (gaining 5 Stars...). With the popularity of playing the game just for the female Servants or waifu, Teach also gets another use for his third skill which heals his team, and double the amounts for female Servants, if you don't mind him being the sole middle-aged man amongst your group of beautiful female Servants.
Mata Hari, much like her lore depicted her, is not a typical assassin that kills others with ease, as shown with her overall atrocious stats. She has no protection, so she can't take much hits, and she has amongst the lowest attacks in the game. Fortunately she makes that up with several things: Her capabilities of debuffing her enemies in both offense and defense or charming them (at a lower rate than Euryale and Stheno). Her QQQAB deck can be offset with equipping NP-generating CE. Her 3rd skill can induce a Skill Seal, against a boss that likes to buff themselves with skills (like the Final Boss), she can find a niche use. It's still a bit tricky thanks to her low stats, but as long as you remember that she's a debuffer and support, not a killer, Mata Hari may see some use in the battlefield besides just looking pretty.
As much as his popularity in Fate/Zero did for many anime fans, Diarmuid is kind of doomed in this game, he's the kind of Servant that has no idea on what he wants to be. He has a dodge skill, a situational debuff and a Star generation rate up skill, all of them doesn't have synergies with each others, and likewise, his stats were nothing to write home about: Decent, but not at all spectacular or focused in certain fields, not enough to be a tank or an attacker. Diarmuid is a very situational Servant that perhaps can have his uses when you know you'll be fighting a lot of females (Training grounds that has Amazons or some female Servants, before late game) and at the very least he has a respectable NP damage and gain, but he's seriously hampered with the lack of synergy of his skills.
They tried to make Saber Gilles a pseudo-Berserker, someone who relies more on normal attacks than NP. But his middling stats, even for Sabers, puts a serious hamper on that. In between being unable to utilize a Buster Chain alone and an unreliable Noble Phantasm (since it's controlled with the randomness of drawing the card), Gilles takes a lot of work to be useful, his third skill locked within the Strengthening Quest only mitigates this a little, and his poor reputation as one of the worst Saber cards are not unfounded.
Boudica is a lower tier Servant that has an almost legendary status amongst players, and they wouldn't be too far off. She seems designed to be a Support-based rider that has a niche skill to boost up her offense against Roman enemies. Those fell into serious problems with how she has bad offensive stats and no NP-generating help and the fact that there are few Roman enemies and her bad offense makes that niche bonus a bit irrelevant, not to mention hinders her Rider advantage for gathering Crit Stars. You will mostly be using her not for efficiency runs, but rather for favoritism's sake.