Mystic code

Personally I think the Combat Uniform is great. The swap skill is almost overpowered honestly. You can use it offensively to bring in a fresh servant to buff your team, or someone that can instantly charge their NP to do a (bigger) NPchain. Or defensively to bring in something like David to dodge an AoE NP, or Mash to tank ST NPs, or simply just to get a member out of the way if you need them for the first and final wave, but they are vulnerable in the second one.

The stun is also great to stall for more damage/ NP charge on your team, and the attack buff is fantastic with NP chain turns.

Go for Combat Uniform as that thing's Attack buff scales with each level upgrade. Also useful for a lot of tough content and the Order Change skill brings a lot of potential to team comps.

Mage Assoc Uniform will naturally acquire EXP if you use it for farming. I don't really use it for much else. The np battery doesn't scale with level. You'll only benefit from more Team heal as you level it up.

Atlas Uniform doesn't benefit much from levels cept for lvl 6 and 10. Tho a really good mystic code for Arts teams and Solo runs, I recommend leveling it after Combat Uniform as you'll only gain lesser cooldowns at lvls 6 and 10 respectively. 15 -> 14 -> 13

Combat Uniform and Atlas Uniform is usually the preferred mystic codes for harder content.

Mages Association is usually used for farming.