Saber: Rama. He is both greatly powerful and super all rounded, being able to battle rather well no matter who comes. Also Charisma boosting.
Lancer: Enkidu. Can pull a Gilgamesh, has even better stats, Presence Detection makes strategizing much easier, super regenerative mud boi, nearly literally unkillable unless there's soul shenanigans abound (plus he's waifu).
Archer: Arash. Pure and undiluted marksmanship for ultra long range support from best archer dude. And things somehow go to heck, all the enemy servants being lined up opens up for some STELLAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA!! fun.
Assassin: Fuuma. It's a goddamn Ninja that has a skill telling you "he's good at ninja shit". Superb presence concealment, ability to reduce enemy numbers with Subversive Actions and potential to abuse it further with Servant companions and his NP, and a skill that if used alongside his NP makes him literally perfect to debilitate the enemy, throw the master(s) into disarray if they find him, and as a last resort he can pull a Jekyll/Vlad and put on his mask, letting the Oni out.
Rider: Ozy for days, because a **** ton of sphynxs and massive complex with traps and super weapons is the greatest way to say "I hate you, please have fun".
Caster: Zhuge Big Ben Pipsqueak Star. The Stone Mace is a small advantage, but it doesn't compare to his 3 skills optimizing the functioning of all the Servants and related creatures invoked by Ozy into an effective fighting force, at the same time that he gives advice that is gonna be 100% accurate. Because Chinese Servants are stoopid silly.
Berserk: Mama Raikou. "Such delicious, big, bountiful Divinity... Now present thy head, swine. It shall make fine addition to my collection."
I wanted to waste some time thinking about this, if it wasn't obvious.
