Best mystic code?

Combat uniform is regarded as the best usually for team switching. On demand stun can be pretty useful in boss fights too.

The other ones are still good but don't have the sheer utility of the combat uniform.

Combat uniform all the way! Haven't used any other since it came out and just got it to lvl 10, yay!

Atlas can be useful for servants that really depend on skills with the 2 turn cool down reduction.

Depends on comp. That's the simple version to it.

plugsuit is not as all powerful and universal as people make it to be.

I strongly agree with Masters above - Combat usually is the most useful. Stun sometimes literally saves your ass while Attack boost and swap help a lot to get 1- turn boost overload
For example:
you use all Waver's skills, then swap for Hans (or another Waver if you got one) with CE that gives at least 50% starting NP gauge and you're able to do both hyper boost and NP chain
of course this is only an example because there's a lot different strategies while using this MC

Sometimes during farming you can find Mage's Association Uniform useful.
For example: Lancelot (or any other servant with low NP generation but with AoE NP, like Arash, Mama Raikou or... Altera? idk) with Kaleidoscope - with this MC and non-MLB Kaleidoscope you're able to get 100% NP gauge on 1st turn

But those are only suggestions based on most common used strategies. The true power of MC lies in your decision and strategy making. You can make MC consider as useless in something truly powerful.
But for now... just try to focus on Combat uniform :D

The Combat uniform is generally the best all-around mystic code you can get from Chaldea Gate, as the combination of Attack buff/Stun/Team switch is invaluable for bossfights.

The Mages Association uniform is pretty good for farming thanks to 20% NP on demand, but doesn't really have much else going for it. While the party-wide heal seems like a great buff, the amount of HP it ends up healing aren't that impressive, and the command card reset isn't really that important in the long run.

The Atlas Academy is great for solo attempts, since it is all geared towards guaranteeing the survival of a single servant. It will be useful for boss fights to protect your designated carry, but ultimately isn't as versatile as the plugsuit.

Ultimately, if you want to run Quick, the best Mystic code for that is the Royal Brand that is available on the Rare Prism shop, but the Royal Brand is pretty unimpressive as whole regarding anything other the quick buff it gives(the other 2 skills are a targetable Sure Hit, and a targetable crit star absorb, both for one turn), so whether or not it's worth the 5 rare prisms is your call. The other Quick mystic code is Brilliant Summer, but it only gives 30% Quick up instead of the 50% that Royal Brand would give if both are at level 10.

Chaldea Combat Suit of course, i used it for farming ( switch to buff and fill NP Gauge ), for Raid ( long battle or short battle ), Stalling, buff stacking and more.
It's best use is in the bossfights and help to clear harder content.

There is tons of tactics you can use with Chaldea Combat Suit.

Best all around choice would be Combat plug suit. The stun is really helpfull and switching pary members is one of these things that you would wish wouldn't be tied to a mystic code
Atlas is also really good if you plan for Cu solos
Mage association can help you charge your np faster and sometimes reshuffle cards is a nice thing to have but I find it better suited for arts teams
On a side note, if you manage to eventually get them Royal Brand and Brilliant summer(slightly better than Royal Brand imo) are really good for quick teams

I like my Brilliant Summer swimsuit. I wear: Combat for serious business, Chaldea for survival, and swimsuit for fun. It's Quick-based.

You should make Combat uniform leveling a priority to make the "Buff all allies" skill more powerful in tough battles. Swimsuit and Royal Brand both don't require nearly as much exp for max level. They are both Quick-based but they are probably not available to you. You'll likely get another chance at them when those events rerun.