I operate an occult and religious items shop for 25 years, and i continually ask my customers which spells work best for them -- and i'm happy to pass along some trends i have noticed in this regard.
In any given situation, what i have learned is that the best spell for YOU is the spell that --
• actually has as its goal the goal that you are aiming for
• can be performed at a cost in materials that you can afford,
• is worked in a way that accords with your family's or culture's magical traditions
• is worked in a way that accords with your personal emotional temperament
• is worked in accord with your own religious beliefs
• does not require you to violate your personal ethics
• does not require you to change your religion
• does not require you to engage in activities of which you disapprove
• has been recommended to you by a competent professional reader or diviner or magical mentor whom you trust
• is historically authentic according to the tradition in which you choose to work
• is well grounded in traditional magical concepts, including those which are known to magical theorists under titles such as the doctrine of signatures, the magical link, the law of sympathetic magic, the hermetic rule, the law of magical contagion, and so forth; your teacher need not use any of these academic terms, but an academic magician should be able to recognize one or more these principles at work in every truly traditional spell.
People often ask me if a particular magical ingredient will "work" for their purposes or if a spell they are casting "will be stronger" or, conversely, "will get messed up" if they use a different ingredient or blend of ingredients.
The practice of magic does not operate according to strict rules such that an outside observer can calculate your "winning odds." Just as important to the work as good, traditional tools and curios associated with the form of magic you are practicing are these factors:
• You must develop a certain degree of focus and dedication, that is, an ability to perform the rite in a spiritually concentrated state of mind and to carry it through according to authentic teachings.
• You must take into consideration the degree of vulnerability and accessibility and suggestibility of the person you are attempting to influence, including his or her willingness to be ensorcelled or his attempts to break your work or protect him or herself from it.
• You must honestly evaluate the degree of harmony between your stated goals and your actual personal behavior; for example, there is not a lot of use in attempting love magic if you are suicidal and also hate sex.
In other words, to use an analogy to cooking -- you can buy all the fancy ingredients in the world, but in order to present a successful dinner party:
• You have to follow the recipe until you learn how to improvise and change things a bit, and to recognize that certain elements in a recipe cannot be changed or the recipe will not work.
• Your dinner guests need to be hungry because a tasty home-cooked meal is of no use to people who have just stuffed themselves with junk food and sugar-drinks.
• No one will come to your lovely dinner party if they know that your house is filthy, you curse and swear at them, you are prone to throwing knives, and you are the one who killed their kitten.
I hope this all makes sense to you and explains once and for all why we cannot actually predict the outcome of a spell performed by someone we do not know on someone we do not know, even though we are the people who supplied the ingredients that went into the spell.