Which ranking do you consider the most useful? I personally prefer the one from the EIU (mba.eiu.com), because it gives a lot of practical information like the ratio of applicants to availanle places, the application fee (if any), average GMAT, and others that really help you to narrow your selection.

Its always worth checking out the FT and Economist rankings, too. Not just their rankings, which also provide various sorts of practical information - they also frequently carry articles on certain programmes and schools. That said, for first-hand experience I think you often get a more personal and more discursive (and potentially more accurate) look into what it's like to study somewhere on here. As another poster on the board recently commented, rankings aren't everything, not by a long shot.