Mystic Codes

Combat uniform is the best one imo,takes a lot of time though

Dunno which all ones you have, but I'd probably say that the Chaldea combat uniform is the next most likely to really benefit much from levels in a meaningful way. Atlas and Mage's Association are both nice mystic codes, but levels don't really do all that much for them, other than change their incredibly long cooldowns into cooldowns that are still basically too long to use more than once per battle.

Memories of the Lunar Sea:
Good for Art team (typically Art critical team and Art NP nuke shot). And good for NP spam of specific servants. However, not so many people have it.

Mage's Association Uniform:
Leveling Value is not high, since it required very high XP to lv10. It 1st skill is decent. Generally good for Art team, typically Stall team. But its 3rd skill is RNG gambling

Atlas Academy Uniform:
Excellent for challenge quest, especially for solo games (Herc, Cu, King Hassan, etc) However, due to it very long CD even though you get it lv 10, hence its leveling value is also limited.

Royal Brand:
It is generally good for Quick team, like quick combo of Okita-dual Jack. Its 2nd skill is golden for specific critical brave chain of one servants as well as for star/NP spam. Also good for servants using Quick NP and NP-x-x-e or B-NP-x-e brave chain. Its XP requirement is low, so recommended for Quick servants.

Chaldea Combat Uniform:
It has shown very unique value of 2-3 skills, which could slow enemy release NP (or even eliminate enemy NP), and save ass for your team. Very good for Art team (typically durable stall team). Like switch it sub member of David/Tristan, Mashu/Geogies/d'Eon, etc.Also very good for critical NPBBe nuke such as Dual Merlin + Waver support Jalter to accumulate offensive buffs then one shoot enemy. Sometimes even better than Anniversary Blonde. Therefore, it provide very versatile and flexible idea for team composition. However, still long CD became very valuable. But general leveling value higher than Atlas and Mage.

Combat uniform all the way. I got it around the time London came out. Played with all the new ones for a while. Then I realized that combat uniform was my favorite and haven't used any other mystic code since. I just got to lvl 9 this week. Will probably be a full year or more to finally get lvl 10.

Combat uniform and Atlas Uniform 1st before anything else.

Chaldea Combat suit is really versatile on many types of battle, such as Stall, One turn kill, buff stacking and more.
For me i use Atlas MC or Combat MC for harder content and Anniversary Blonde for shorter battle.
I have Anniversary Blonde at lvl 10, but the Combat suit playstyle offer is just a lot more.

Chaldea default is also considered the best MC, combining straight forward yet simple kit and can also be used by a lot of teams or set up.

I would say level each one of them naturally, since most MC needs monstrous EXP.

Aniversay Blonde is sooo good for Buster crit team, unfortunately I arrived too late to get it