DECIDING ON A NEW MYSTIC CODE

Imo Chaldea combat uniform is the most useful mystic code. 30% attack up for all servants, stun, and the ability to switch out servants (which you can do to switch out for a taunter which can save your team)

I recommend the Anniversary Blonde Mystic Code. Seems like you like Buster Oriented Servants. It has a Buster Up, Gain Crit Stars and 1 turn Guts. A shame it doesn't have a heal since it is for bursting damage in one go. I already have mine at lvl 9. If you have missed that, I think the Chaldea Combat uniform is a second good choice. Though I gave up on that, hogs too much exp. The Mage's Association is another option since it has a team wide heal, 20% NP gauge and card shuffle skill. Perfect for farming. You can choose on those 3.

Honestly it seem's like you have a somewhat similar servant grouping as me. Personally I use Anniversary Blonde. It works wonders for buster crit teams. Raikou, Karna, and Gawain in particular should become absolute monsters with a maxed out Anniversary Blonde, since it boosts any buster cards of the selected servant by 60% and gives you 20 crit stars at max level. The only downside is that the only defensive applications it has is a 4000HP Guts, which is harder to use than a dodge or invincibility, but with good timing, it can be a life saver. If you don't want to go with a buster crit monster team then you could try for the Atlas Mystic code since it basically revolves around making your team survive as long as possible and spamming servant skills as often as you can. I personally only use it when I'm trying to do a solo run with Herc or Cu Alter since it makes them even more durable then they already are. You're other default option is the Mages Association Mystic code, which is basically for the people who want a challenge in my opinion since it offers essentially no bonus survivability or attack. Though it does have the cool ability to reshuffle your command cards so you can possibly get the combination you want. Hmm other than that I really don't use any of the event exclusive Mystic codes because they don't really fit with my play style and don't synergize too well with the servants I prefer to use. So that's about all the advice I can offer. I'd personally go with Anniversery Blonde. I've gotten some ridiculous damage with it lol. Highest I've ever got was like 2.5 million, granted that was with even bonus CE equiped, but outside of an event I have occasionally gotten close to a million. When Merlin comes out it'll be super easy to get a million plus with a maxed out Anniversary Blonde Mystic code and a good Buster Crit servant like Raikou, Jalter, Gil, etc.

quick comment of what they're for:
chaldea combat uniform: allows for buffstacking and more aggressive plays.
Mage's association: good for slow fights.
Atlas uniform: good for solo fights
Anniversary Blonde: RIP AND TEAR THEIR GUTS! (crit buster's wet dream)
Royal Brand: more like "sodding bland"
the Swimsuit: the what now?