It all depends on a few factors.
heres the map of the site
1, Do you want to get sleep if so aim for outlaying sites like Wicket field. Lime kiln, Hitchin Hill, Paines Ground as you can see these are on the extremities.
2. Do you want no sleep or ear plugs needed then Big Ground,Home Ground, Kidney Mead and Dance Field are for you party Central close to main shopping,bars and food area's
3. By Wed lunchtime most of Dragon field, area's around Pyramid will be close to bursting.
We always camp in the Wicket Ground as it is close to exit A where we alwayd seem to be directed to park. But you never know at Glasto you may get put further down close to Exit D
Also depends where you are travelling from and which side car park you go to West side exits A and D East side B and C. You may find yourself carrying your stuff over 2 miles if you decide on a spot now and find yourself parked on the wrong side of the festival.
West side Car parks are for people coming down M5 and East side are for poeple coming from A303 etc
hope that helps any more info then ask away