I have an iMac desk top version OS X10.6.8 I want to upgrade to the latest version there is. Do I need mountain lion or snow leopard.

You already have Snow Leopard. The last version of Snow Leopard is OS 10.6.8.

Lion (OS 10.7) is no longer available from the App Store.

Mountain Lion (OS 10.8 — current version is OS 10.8.4) can be installed on Macs newer than 2007-2009 (the exact date depends on the model of Mac, when certain hardware was included inside each model). Make sure you meet all System Requirements before purchasing and attempting to install.