What is a good delivery format from a server to a utility app?

What Format should data be in when going into a utility app from a server? JSON, XML or something else?

Edit:

I need to tell a client to tell his web guy to b开发者_JAVA技巧uild me a web service or let me do it. I need to make sure I am asking for the right thing.

The best format is Apple's Plist format. The iPhone OS can quickly and easily parse this. It's faster and less memory intensive than XML.

Best of all, it requires no additional libraries or parsing code on the device.

There are good libraries for parsing and emitting both XML and JSON in Cocoa Touch. I recommend KissXML and either Stig Brautaset's JSON or TouchJSON.