Hi. I'd like to know if there is a relationship between the grade or
quality of loose green tea to antioxdient activity.

A survey of the vast number of green tea brands available at a
specialty store in Chinatown showed a considerable price range...the
most expensive was around 3x the cheapest. I am told the price depends
on the tea grade. The more expensive ones are said to have better
taste and able to extract out more of it. I have had green teas that
were nearly like water, and others that had a dark color and rich
taste so yes there is a difference in taste.

But I don't care too much about the taste; I am only interested in the
antioxident property.

Should I go with the higher priced ones that have more flavour because
they also have higher antioxident level or just get the cheap one
because all  green teas have about the same amount of antioxident?