Which kind of lavender is the most common?

Depends on where you live. Lavender is a mediterranean plant that needs certain soil conditions and climate to grow well. In England, you might find more of the "English" variety, although it did not originate in England. It is native to France into the Italian alps. In Bulgaria or Turkey, where some of the oil available for puchase all over the world today originates from, "Vera" is often the most cited variety. Lavandula agustifolia has many types used for different things. If you want to use it in your cooking, or to flavor drinks, use the English varieties. Some common forms of this that you will find in garden centers are "Vera", Hidcote, Munstead, make sure to use only culinary grade lavender if you buy the cleaned lavender flower buds for cooking. Lavandula Agustifolia Intermedia "Provence" is a hybrid that is also used for cooking as it has a milder flavor than some of the other lavenders used for oil. -an Excellent text to do more research is The Genus Lavandula by Tim Upson and Susyn Andrews