Is this just for the FSN and FZ servants? Because an answer could go on forever if I included all of them from Grand Order.

For Fate Stay Night

Arturia Pendragon: Saber (She can be summoned as a lancer with Rhonymiad but because of Excalibur's unique nature saber is the optimal class for her.

Emiya: Archer (I'm fairly certain this is his only class.)

Cu Chulainn: Lancer (Can be summoned as a Caster and Berserker but his lancer version is his prime for his skills like protection from Arrows and Battle continuation.)

Medusa: Rider ( Can also be summoned as a lancer... I'm actually not certain with this one.. But I believe bellaphron's nature as a divine beast is superior to Harpe.)

Heracles: Berserker (Can be summoned in every class except caster. It's been repeatably mentioned that Archer is his strongest class.)

Medea: Caster (Her best and only class is caster due to her nature with hectate and Circe.)

Gilgamesh: Archer (Can also be summoned as a Caster. Archer is his optimal class.)

Sasaki and Cursed arm Hassan: Assassin. (Only class they both fit into.. Their best.)

^ I believe that's all for fate stay night servants. I might do Zero in a bit or any other servants you request.