Personally, I like to make such decisions on the visuals as, in lots of cases, faction guns can be quite comparable. They may do less damage, but are more efficient. That said, I think SILVERFIRE are the bestest because they look nice, have a cool sound effect, and sync up nicely with the Vultures the ship using it has.
So, really, I'd say what you'd probably want would depend on the other weapons you're using to see what would complement them the best.
For the Order ship, if they're being paired with the Order neutron guns, I'd say something like an IRON HAMMER would be good - it would sync up and the purpley color would complement the blue of the Order guns, assuming I am remembering their color right.
I have 3 ships that use codes, and they're all different. I'd say the ARCHANGEL is my favorite, but here's the thing... you gotta have good aim to use codenames, as they're slow in both refire and projectile speed. Not much of a problem for me, but maybe for you... try mounting up civilian/Zoner guns first, get a couple of contracts, make some allegiances, then use the guns of your new employer.
Depends what you're looking for. One thing to remember is that there are a couple codes that are rather different from the others. The Golden Blade, and the Solaris are two in particular. Solaris' have a 4.00 refire, more than that of any other code, but are rather low on the damage side. Golden Blades have a 650 projectile speed - 50 more than any other code - but are otherwise mediocre. As for the rest, I'd recommend finding a decent looking one in the refire you want. Personally I like the Lazurites, Hammerfalls, and Delusions out of the 3.03 codes. Not a big fan of 2.00 refires myself. Really, though, it's up to you. Oh, and some faction class 9s are better than codes. Krakens and Buckshots spring to mind. Remember that a good projectile speed is rather nice, as it makes hitting things quite a lot easier.