Green grapes are either late or stored varieties from South Africa and Chile, or Thompson Seedless from India. Thompson Seedless is a good grape, a mainstay of many producing countries for many years, but relatively unexciting. For the best, look for a lovely light-yellow straw colour.
For something a little different, look for Autumn Crisp and Sweet Globe, both with nice round berries and wonderful, crisp, firm texture: delicious!