I use Myxazin, as that's the medication I can get most easily here in Aus. It's pretty effective, I guess close to Kanaplex and Furan-2 as far as that goes.
From the research I've done, it seems Maracyn is good for catching disease at its beginning. Problem is, columnaris can advance slowly and look like a bunch of other things (sbd, true fungus, etc) and then hit really hard when the fish's immune system finally crashes. Or it can just hit hard right away internally (septicemia), or it can cause rapid necrosis, etc.
So I don't mess around - I use the gung-ho antibiotics and knock it out fast, having lost a fish before to dithering with salt and whatever. I've managed to keep one sickly and very prone to infection fin-biting fish alive for almost a year by smacking any sign of infection with Myxazin. These days, he doesn't need that anymore, just clean water and good food, now his immune system has built back up a little bit.
There's a couple of diseases that look very much like columnaris (aeromonas, pseudomonas,
vibrio, saprolegnia). They tend to also be gram-negative bacteria however (except sapro, a true fungus - this loves to grow on necrotised flesh caused by the g-neg bacteria, though, so its symptoms, 'fin fuzzies', etc. can be an early warning sign) -- and because they are g-neg diseases, Maracyn 1 and antiseptics like the -fix meds won't affect it - they are for gram positive diseases like staph.
But a huge proportion of bacterial fish diseases are caused by gram-neg bacteria, so it's first port of call to me to hit the disease with a strong, effective medication at first symptoms. If that doesn't work, you can count out the g-neg bacteria which significantly narrows the diagnosis field -- you're left with g-pos bacteria or parasitic infestation. And hitting it fast means you might still have time to try other medications before the fish dies...
Some detailed info on columnaris
Some detailed info on aeromonas, vibrio, etc