Start with the weakest one. Use that one for a while, or until you and your meister are in an intense situation, because that seems to be when people unlock hidden talents. At that point you can upgrade to a higher form. If you're a scythe, you'll probably use the scythe techniques from the show and so on. Power wise, 1st technique is the weakest, 2nd is medium power, and third is your most powerful.