Which green teas are high quality?

Many consider gyokuro to be the highest-quality Japanese green tea. This is, however, a matter of opinion and there are no rules.

About 75% of all tea harvested in Japan is graded as sencha. Sometimes described as a common tea.

Sencha

The quality of sencha is, however, highly variable. Quality depends on the tea's origin, the season, and leaf processing techniques that are employed.

For gyokuro only the youngest flush leaf is picked. Special cultivation techniques are employed to produce a tea that is famous for its rich flavor and pale lemon-green color.  

Gyokuro tea plants are grown in the shade for approximately three weeks before harvesting begins.  Removing direct sunlight in this way reduces leaf photosynthesis which alters the proportions of flavenols, amino acids, sugars, and other substances that provide tea aroma and taste.

For high quality gyokuro the leaves are (unlike sencha) aged for about 3 months. This aging blends and mellows the flavenoids and other organic compounds to produce a tea that is renowned for its smooth elegant taste.

This wonderful tea differs from gyokuro in that the leaves are not rolled at all. After steaming, the leaves are thoroughly dried. This is tencha. The tencha is then ground into a superfine powder, and that powder is what is known as matcha. 

Matcha is delicious and energizing.

Premium matcha is very finely ground and whisks to a brilliant green color.