Who should I focus on max leveling?

Depends on who you already levelled and what you want to do, but in general, I'd say, in order:

- Ibaraki Douji: good ST berserker with decent staying power, can serve as the main DPS in a variety of teams. Skill levelling: first > third > second

- Jeanne: having a ruler is always good since they resist damage from most enemies. She also gets really good after strengthening in 2019. You don't really need to level her skills, they aren't why she's good, in fact they are all pretty bad. But if you really want to, I'd say first > third > second

- Robin Hood: good nuke. That's basically it. Use him to deal with sabers. Skills: first > second = third

- From here, I'd say level whatever class you are lacking. Caesar and Bedi are both good sabers, level who you want. Caesar works as support while Bedi is all dps. For assassin, probably Scathach, and for caster, idk if you've levelled Hans already, but if you haven't you should. Also Rhyme is pretty good.

- After this just level who you want. If you really want a guideline, I guess do the farming servants first (Arash, Spartacus, Siegfried, etc).

Honestly, depends on who you already have and where the holes are in your team. And who your waifu/husbando is.

I don't like Martha rider, np is weak for a buster.
Siggy is a bit weak, but will get awesome strengthening later.
Scathach assassin is decent star generator and the AoE np is great for farming.
I really like von hohenheim because of his skills despite the long cooldowns and very spammable np.
Kiyohime hits too weak for a berserker.
Robin hood's np is a nuke if you can poison the enemy first.
I don't use cu proto or caster much anymore, especially now I have liz caster for a buster caster.

I don't have the others so can't speak for them.

In my personal opinion, you should definitely level Jeanne D'Arc. I have her, and she has been really good for me. She is, thus far, the only servant I've grailed (I've grailed her up to lv100).

First, raise your servant based on the AOE and ST Class advantage.
See if you have the hole or weakness in there.
Pretty much from my view, you have a lot of strong ST, needed to add a little AOE for Wave Clearing and Farming.

Servant like Arash, Spartacus and Shakespeare is essentials for 3 turn daily farm.
Bonus : they also have strong NP against Large HP mooks and if you've got some experience and skill in team building, Arash and Spartacus is a meme for Boss killing.

Also don't burn the 3 star CE like Dragon Meridian/Leylines, event CE with 50% NP gauge starter is also recommended.