Apart from some ST boss-killers others have already mentioned, I agree that Helena and Nitocris are two of the most important SRs to have.
Helena is the only non-SSR with team-wide 20% NP charge afaik, and her 3rd skill is quite versatile, plus if her 2nd skill’s NP buff triggers, she can output rather high AOE damage herself, even at NP1.
Nitocris is the only AOE servant with a skill that charges her NP to 100%+, and with her Death mechanic she can kill some mobs that would otherwise have too many HP. She’s also a prime candidate for CEs like Black Grail, and she can do back-to-back NPs with a KScope + Helena.
Otherwise, at least for farming, getting an NP2 SurferMo from one of the DL campaign banners next year would be useful, too. With a Helena with 10/x/9(or 10) skills + Paracelsus with x/9(or 10)/10(upgraded) skills, she can easily NP loop any node with 3 casters per wave. And her star gen + tankiness are OK too, so its not like she’s useless outside of farming.
And of course, don’t neglect the better ones amongst the 1-3-star servants.
Robin, David, Euryale, Caesar, Medusa, Medea, Arash, Spartacus, Paracelsus (at the latest when he gets his skill upgrade), Bedivere… there are many good ones, and some of them definitely worth grailing up by 10 levels, too, making them perform as good or even better than several SRs.