The Gel Tek products are fantastic. As MrVampire said they pack some serious muscle. The Maracyn products are is mid level antibiotic and bettafix and other more holistic medicine are best for mild cases.
I'm a huge fan of the holistic approach. anytime i've had a sick fish unless it is a scaleless fish I always begin with a salt bath then return them to a cleaned tank with either a light dose of bettafix (for anabantids) or melafix for other fish as a preventive for the illness spreading. I have had great success with this, but some things are far to strong for the holistic approach, thats where the stronger meds come in ans Gel Tek is as good as they come.