Levelling up suggestions

I'm assuming by Mend, you meant Medb, and Mordred is the summer rider version.

The best servants out of those, in order, are: Chloe, Mordred/Tristan, Medb, Scathach, Serenity.

However, while that is the order individualy (in a vaccuum), Mordred, Chloe and Serenity are all arts servants, so they have some synergy together.

The answer largely depends on what classes you are lacking in. Regardless, unless you have a top tier archer like Altria or Gilgamesh, level Chloe first since she is a largely independent arts ST archer with good NP spam.

If you were playing during Onigashima and have rider Kintoki, level Scathach assassin unless you want to use serenity in an arts team. Overall, neither are particularly great, but Scathach has better stats due to being a 4 star and her NP actually does damage.

Finally, either Mordred or Medb are decent options with different pros and cons. Medb is a generally weak 5 star, but is a 5 star nevertheless, so without other great 4 star riders like Kintoki, she is still a worthwhile investment. Mordred is generally considered good, but that is largely only for farming, and even then she is only at her best with the proper arts support (waver, tamamo) and without them, Mordred's performance is pretty underwhelming. Either one of them are fine, pick whichever one you want.

TL;DR: Unless you have a really good archer already:

No better riders:
Chloe --> Medb/Mordred/Scathach (doesn't matter which order for the last 3)

With better riders:
Chloe --> Scathach --> Medb/Mordred

Since you're already so far ahead with Mordred and seem to have arts servants mostly, imo just stick with her.

Of the remaining ones, for gameplay purposes imo Tristan is the only one worth levelling. He is a solid archer, and the only quick ST archer. Serenity is okay, but requires extensive support to be worth it. She is largely an enemy debuffer: the best thing about her is a reliable NP drain. Otherwise, she's pretty substandard in terms of damage and durability.

EDITED for new info: okay so I guess you also have some casters lol
Both Medea and NR are good. NR is AOE with high durability and a weird self-crit boost, Medea is an ST (with AOE damage) with enemy debuff capabilities. Both are solid and you'd be fine with either one.

Personally, I’d go with Mordred

But who do you like best is the big question

Prioritize Chloe over Tristan. She's on a higher lvl and she's at NP 5 if you can complete the event. I think you can make a Decent Arts Team so you can also use Hundred Personas, Mordred and Both Nursery for Farming and Medea for bosses.

Gonna keep it simple. Till you get 3 max level servants, level whoever has the easiest available ascension mats. That means Chloe and any other welfare SRs take priority if you managed to collect their ascension mats. Mash is unfortunately tied to the story as far as her ascensions go so you'd have to play through the story to ascend her. After these servants are max levelled though focus more on quality of servant over ease of ascension. To ascertain servant quality just look at the tier lists here and read their individual profiles to find out what each servant is good at.