Welfare servants

It isn't exactly answer to your question but we're going to have Onigashima re-run in June so you'll be able to get Kintoki Rider

of the ones we've gotten so far
Chloe's the best archer welfare
Kintoki's the best rider, (for boss killing as he's ST, for farming santa's better) we're getting a rerun
Caster welfare I'd say liz, but I don't know how good the new Shuten caster JP got is
Shiki is best welfare assassin, but we're getting a rerun for summer scath next year so you can still get her
Brave Liz is better than saber lily

As for 3 star servants who can do the job fine if you missed the welfares without reruns/don't have 4-5 stars for the class.
Fumas a terrific 3 star assassin even if not ST
Medea is a good offensive caster
Bedivere and Caeser are good sabers
Ushi is a perfectly usable ST rider, she can't match him in card damage output but at NP5 even though his NP5 hits harder hers is no joke.

Kintoki rider will come back. Chloe won't, but we will get nobunaga again very soon.

I'm not quite sure what the question is. If you are asking about the best welfares in Kintoki and Chloe's classes, then I'm sorry to say they are both the best in their own classes. For archer there is also Nobunaga, and rider has Ishtar and Sakamoto, but none of them are as good as Chloe or Kintoki. In terms of the best welfares in each class, imo:

Rider: Kintoki
Archer: Chloe
Caster: Sieg (not by much tho, none of the welfare casters are that great)
Assassin: Shiki
Saber: Liz brave
Lancer: Jalter lily santa
Ruler: Quetz (only one)
Berserker: Chacha or Jalter
Mooncancer: BB (Only one)
Alter-ego: Both are good (Mecha-eli chans)

Rider: Kintoki for Quick ST, await for the rerun next year (Ushi post interlewd and NP5 still hits like a silver truck), Santalter for Buster AOE, rerun has passed (try investing on Blackbeard...that's not a joke, his Voyager of the Storm maxed is quite the AOE nuke...)
Archer: Chloe for ST (best welfare, you can use Euryale and David as Arts ST nukers as consolation), Nobu for AOE (march 2019 is the rerun, if you fail, Kid Gil is a ok Buster AOE, but do hi interlewd to make his NP dmg non-shitty)
Caster: Sieg, the others are kinda bad,, with unmatching deck...maxed Hans>Castelli (even generating less stars), Sheba>Shuuten (aside from the easy NP5...)
Assassin: Shiki for ST (substitute is Kirit just if you get lucky and NP5 him, he can work as a arts ST assassin who ignores defenses in the NP and has pierce invulnerability skil...), Scat for AOE (NP5 Kotarou is a good substitute, better stargen and many debuffs, just weaker)
Saber: Liz brave (roll story gacha untill you NP5 Bedivere as substitute and grail him =P...or get a Rama)
Lancer: Jalter lily santa AOE, no ST Lancer wellfare so far...just this one
Berserker: Chacha, check out her 3rd skill cooldown and duration...

Saber : brave liz (single target with np steroid and good survivability)
Archer : chloe (np spam arts archer with high hit count)
Lancer : alter lily (tagetable heal and np battery)
Rider : kintoki (quick monster)
Assassin : scathach (good star generator with taunt and targetable heal with damage cut and np steroid), shiki (single target arts assasin)
Caster : liz (good survivability and star generator with buster np)
Berzerker : jalter (mana burts with np battery for single target np)
Other : mash (best kouhai)