1.which modeling technique is the best technique to create 3d games?
2.i found most game developers use polygonal modeling, i also found some that uses NURBS.which one is better?
3.is polygonal the most commonly used modeling technique for 3d games?
3.why most game developers uses polygonal?

Polygonal modelling is the name of the game. Mainly because of constraints set by the developers and hardware limits although that is being pushed along at a rapid rate. Just look at things like ATI's TrueForm feature in their newer cards. Never really noticed any that use NURBS before, but it's entirely possible depending on what you are doing and how efficient you are.