Must Have Low Rank Servants

You should check the tier list, it gives some insight about servants
https://grandorder.gamepress.gg/13-star-tier-list

Personally I'd recommend Ushiwakamaru, Cu Chulainn (both proto and fsn) and Robin Hood. Ushi and Robin are pretty good to tackle bosses and fsn Cu is used even in hard boss fights because of his survival kit, proto Cu is a little more offensive though.

As for support hands down Hans is awesome, since I still don't have something like Weaver or Tamamo I still use him a lot and I love him.

Honorable mentions for Spartacus and Arash, they are good for farming and cheap to invest into.

You can follow the recommendation of 1-3 Star Tier List from gamepress (just on the left of this web page, Rankings--> 1-3 Star Tier List ).
Except tier 1 Mashu and Hans are definitely recommended to 3 lv-10 skills in any circumstances, most servants in tier 2 are deserved to raise their skill to lv 10, particularly if you are lacking of SSR/SR. Some low star servants in Tier 3-5 are also need raised and maxed special skill.
Off course it depending on how much SSR/SR you have, which means whether low star servants (except tier 1) could be auxiliary or core of your team are quite various for different players.
For instance you can use grailed Robin with high-leveled skills if you dont have any decent SSR/SR archer, but if you already have bunch of SSR/SR archers, then David/Euryale are more versatile option to raise.