Irrigation - Automation Valve
It is good to get a valve with a flow control so you can adjust how much water goes to a particular zone. You only really need a 3/4" diameter valve, but it is more cost effective to use a 1" valve even though you will likely need 3/4" FNPT to 1" FNPT PVC bushings. A large diameter valve is probably also slightly more reliable.
The Rain Bird brand has slightly better user reviews than the Orbit brand, but whatever brand is used you need to plumb it to be replaceable if it fails.
1 in. In-Line Sprinkler Valve with Flow Control - Rain Bird
http://www.amazon.com/Rain-Bird-CPF100-Automatic-Sprinkler/dp/B00004RAB8 $17.57
