Can anybody please let me know this?
It would be Mountain Lion 10.8.2.
Look here for your specific system
Sorry, pasted the wrong link - see mende1's post below for the right one
As you have a Mid 2011 iMac, the latest OS X you can use is OS X Mountain Lion > http://www.apple.com/osx
What is the latest OS X Server that Apple are selling for £13.99 for then?
The upgrade from ML to ML-server.
The latest OS X Server is OS X Mountain Lion Server, that can be purchased in the App Store after upgrading to Mountain Lion
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