Of course the four leaf clover is the most common of all good luck charms!
It is just as efficient dried in your wallet as it is represented as a jewel around your neck. Nevertheless the most efficient is the real one you find yourself. It is recommended in particular for prosperity and getting rid of bad luck.
The virtues of the four leaf clover comes probably from the fact that it is rather rare and also that it has the form of a cross. But it had already, before the spreading of Christianity, some 200 years before Christ, very high value, as the druids picked them at the same time they went fetching their so cherished, Mistletoe.