In folk medicine garlic has been said to cure just about everything from the everything from the common cold and flu to the Plague!
Some of the old stories of garlic's healing properties have doubtful validity, but many of its claimed health benefits have been backed up by modern scientific research. There are two main medicinal ingredients which produce the garlic health benefits: allicin and diallyl sulphides.
Garlic is a sulphurous compound and in general a stronger tasting clove has more sulphur content and hence more potential medicinal value. Some people have suggested that organically grown garlic tends towards a higher sulphur level and hence greater benefit to health. Whether or not that is in fact the case, in my experience it certainly has the best taste.