One of the rarest and most flavorsome green teas in the world is Japanese Guyokuro (pronounced "Guyo-Ku-ru" with the "Guyo" sounding more like a rapid "gooey").
Twenty days before harvest, the tea plants are covered in a thick cloth starving them from sunlight. In response the plants completely changes their leaf chemistry resulting in a completely different flavour.
Even more interestingly the change in chemistry results in a boost of chemicals called catechins. Gyokuro, in particular, has extremely high levels of catechins and has been studied extensively by scientists who have found that the particular catechins found in Guyokuro inhibits the growth of cancerous cells in humans. There is sound scientific evidence showing inhibition of leukemia, renal cancer, skin cancer, breast cancers, mouth cancers and prostate cancer (see note 1).
Guyokuro is also steeped at a much lower temperature than most green teas (60-70 C) and can be re-steeped 3-4 times without much loss of flavour intensity. The resulting liquor is fluorescent green in colour, herbaceous in flavour with strong umami (marine) tones.
You can order your Guyokuro online here.
Note 1: "Foods that Fight Cancer" by Professor Richard Beliveau and Dr. Denis Gingras is an excellent introduction-level book and looks at the anti-cancer properties of green tea in particular.