You can not go wrong with any of them. The mkI was the only 720p version, but as most stuff transmitted still is 720p or 1080i the need for the later 1080p can be debated. With the mkII came the 1080p panel in June 2007 (type no 9720-9730). They both came standard with the HDR-2 and the BeoMedia 1 was an option. In March 2008 came the mkIII (type no 9760-9772) with ACM (automatic colour management). In November 2008 came the current mkIV (type no 9773-9784) with the D10 panel and BeoSystem 3.1.1 and later (August 2009) the type no 9785 - 9796 with the D11 panel.
If 1080p is VERY important to you (there is a difference when used with a 1080p bluray-player, but hardly any for cable/sat tv) I would go for the mkII or a later version.
You should check the milage (hours/days on). The later 1080p panels can suffer from burn-in, something we belived was sorted out years ago, but there is a fix/remedy available in the service menu to cure almost any case.
Roger