Best way to get data from a Server to Android phone

[bigloof]

I would like to write an Android application which communicates with central server written in php , communicate with it and get/send data, for example user authentication, display of user preferences etc.My question is what is the best way to do it. Use a webservice at the server side which returns an xml and parse the xml at the android or use json , or use a html post response back and parse it.Any help on it would be greatly appreciated.