Best servant for each class

Caster: Focus on hans since da vinci is only really useful for farming in most instances and if you have k-scope and aoe berserker can accomplish the same thing.

Berserker: Nightingale if you can get her 3rd skills to at least level 6 if not higher and if not tamamo cat for farming.\

Lancer: Stick with cu and work on leveling up his 2nd skill, ideally all his skills should be level 6 for the cooldown reduction. Don't worry cu is above most 4 stars simply because of his survivabilty.

Archer: Work on robin hood he's the best 3 star archer for most cases except against male sabers which euyale beats him. Atalanta's main thing is generating quit stars with her np she's really not better than the other 2.

Saber: Deon is a good tank, but for damage and support Caesar far outclasses him..?, her...?., "it"? Yeah it.

Assassin: Go for fumma since he's both aoe like hundred faces and also generates stars for teammates to use.

Hopefully I recommend the correct servants if anyone has different opinions, please be free to share them :D.

Idk wht other options you have for those first 4 classes you mentioned so I'll assume those you've mentioned are the best you've got.

Archer: Both are good in certain situations but also have drawbacks. Atalante is quick-based with good hits on her NP and quick/extra cards so she can generate stars pretty well, but has trouble generating NP. Euryale does massive damage against males but isn't amazing outside of that niche.

Saber: Deon is a tank and that's basically it. If that's what you need, use her. If not, use Caesar. He's the best 3* saber.

Assassin: Fuuma. Best star generator who isn't Jack.

Edit: Just do what the guy above says lol

Are you a relatively new player?
Da vinci as she is more powerful alone
Nightingale is great, but tamacat is better for farming
Rider focus on drake as bonny requires more precise play. Or use ushiwakamaru as she is hybrid
Cu is fine. Try for cu proto as well
Atlanta if you have kaleidescope. Robin for damage david for support otherwise
Caesar
Both fuuma and hassan can work. Depends on your playstyle. Try out friend support and see whcih you like better.
Just use strong servants with class advantage for now. You need more servants to consider team comp.