Egon,
You'd have to tell us more about what you need it for before anyone can recommend a "Best". For example, I use RANT 2000 for basic IF Procedure instruction (but it's not really a "Simulator") and Jepp FlitePro for a more advanced and realistic (though light a/c based) simulation. Do you want emergencies, realistic handling, etc, etc? So it depends on a/c type, stage of training, intended use, etc.