Indeed, the sweetest lavenders are select cultivars of English Lavender (Lavandula angustifolia var. angustifolia), In truth, native to southern France and a small part of northwestern Italy. Fine lavender oil is obtained from the flower heads, not the leaves.
Many people are actually surprised by how sweetly scented English Lavender can be. Most people are much more used to the scent of Lavendin (Lavandula x intermedia) which is the lavender more commonly used to scent soap, candles, and the like.
To be assured of an oil with exceptional characteristics you might want to inquire at a specialist nursery. However, if you find a lavender with a scent that fully delights you, your search may be over. The nose knows. Lavandula angustifolia 'Hidcote' is a commonly available cultivar with a very fine scent. Of course, always smell it to double-check, as cultivar confusion is rampant.