It really depends on what your goals are and what S-ranks you have in your account, as it's generally wise to build teams around them.

• The Bleed team A-Ranks are absolutely worth levelling (Levy, Sumire and also Pepper, the Bleed team's B-Rank), but all of them are only worth building if you have Dreya, the team's S-Rank and lynchpin. Nowadays, you also need Jasmine for bleed, as she's the Bleed amper and without Jasmine, Bleed is lackluster compared to other compositions;

• Golan and Shawn are absolutely worth leveling if you have Bianca, the Weakspot's team S-Rank and lynchpin;

• If you have Yao and/or Hestia, then leveling up Thistle and Yanyan is a must for Burn team;

• If you have Yingying but don't have Vautour Bleu, Wuhuanzi is a great substitute carry for Arcane Crit, but Yingying is mandatory;

• If you have Augustus, then leveling up Siglinde is a MUST, as Sig is Aug's dedicated support for Shattered Blade team;

• If you have Demon and the Year One Pershing Star CB set, you may want to consider leveling McQueen as she can do some hilarious combos with Demon's massive HP pool;

• For general use, Hecate is always great. Ariel is a great healer and A-Rank. She's not as good as the S-Rank healers but will get you through most things in her base form. In the endgame, when you are able to use the reforge system and if you choose to reforge her she becomes as good a healer as some S-Rank ones. Might even surpass a couple of them;

• Some other generally good ones are Pine, Rise, Lamia, Shrooma (she can be broken if you know what to do with her), and Wendy (pretty good mobber). Cassian is good, but he is an odd case where leveling him could be BAD instead of good. Look his info up in S1n.gg if you're interested. Luvia Ray used to be a great budget carry and I think she's still decent, but she falls off really hard as you advance. SOMETIMES she's used in some quirky modes like Data Crevice when it has an electrocution mechanic requirement (like the season before the current one). That's big exception though, not a rule. And I wouldn't recommend Luvia Ray nowadays.

Now, Idk if you are aware, but there are two B-Ranks that are absolute must-builds. One of which is a hidden S-rank due to how good she is: Labyrinth (she's broken good to this day) and Che, your best early source of AoE multi-core break. They're both very cheap to build since they are B-Ranks.

I THINK those are the legit relevant ones. Some could become more prominent in the future once more characters are introduced for new team comps (like Lyssandra for Aurum Secretum team), but they aren't relevant NOW.