I think it's fair to say that the very best green teas com from China; after all they've been producing them for hundreds of years.
The bedrock of China's greens are teas made for the masses:
Gunpowder, Young Hyson, Chunmee - all excellent teas and economical.
Another tea with a long history made to exacting requirements in a staggering number of grades is Lung Ching -
a great favorite around the world with good reason. 
The highest of grades are very expensive (starting at $50 a pound wholesale).
There has always been a great demand for high end green tea in China. Happily, they share some of their treasures with us.
The charming
tiny snal-like leaf of Bi Lo Chun is one that is gaining favor in the U.S.
Other, lesser known greens that we have been fortunate enough to get, are Huang Shan Mao Feng, Tianmu Qing Ding, and Tianmu Yunding - all highly recommended.
Try to buy your greens soon after the spring harvest in May and Jun. Keep them in airtight containers. They will last for months without breaking down. Fortunately for us, so far away from China, adding a little more tea helps when they get a bit older.
Click here for our Japanese greens and here for out Indian greens