Arthuria (saber ) or elisabeth (brave)

Boss? Liz .Farming? Saberface. :)

Arthuria, of course. Its not about stats or rarity, its about having the king of knights x)

Go for both of them (as long as you have the materials of course). It's always good to have at least 1 servant with an aoe np and 1 with a single target np for each class since they fill out different roles.

You lacking in boss slayer ? Go for Liz
You lacking in farm/wave clearer ? Go for Arthuria
Raise Liz/Arthuria based on your needs or choose your favorite among them both.

There's always good saber *cough* Gilles *cough* and the only class with the lowest rank of silver yet.

Fergus is very strong Silver saber, he is toughs and hits real hard ( He can solo Camelot Final Boss and lots of strong Lancer bosses with ease ). FGO even give him nickname '' Fergus Kiwami '' to him. A Fortress like Saber, who's Attacking and Tanking at the same time.
And also a Solid Farm/Wave Clearing Servant.
Similiar playstyle with Gawain.

Caesar is good buffer and a strong ST saber, you can't go wrong with him ( he is one of the best 3 star ST saber along with Bedivere ). Just see his kit dude, there's only Damage steroid there.

As for Bedivere, he is still the best of 3 star ST saber ( if you can afford him higher than NP 2, then he can slap even harder ). Rival of the Faber.

For the quality, i would say they are premiere servant.
Even better than some 5 star and 4 star in terms of gameplay.

I only add up a little explanation, because we already have good sabers ( maybe they have weird face or looks but don't judge a book by it's cover ).
For the servant choice, it's up to you.

I would focus on Artoria first, farming is what you most of the time and a lot of people can become boss killers with proper setup.

Artoria, for example, was perfectly capable of one shooting Cu and Scathach on the Nero Fest quest with Excalibur and some crits.

Focus on liz first since she get all her ascension from the shop so she only need embers and being np5 single target buster saber making her good lancer slayer, while artoria good for farming you have to babysit her because she doesn't have defensive skill like liz