The best material, level of insulation, and grade of insulation wholly depend on the temperatures you will be most of the time. When it comes to choosing the best material, I usually recommend pre-treated synthetic material or leather.
For autumn and spring mild cold temperatures, ice fishing boots with a 200-gram level of insulation is sufficient; however, for freezing temperatures, I would recommend at least 400-gram insulation.
Higher grades of insulation would make your ice-fishing boots heavier. So to make sure that you invest in the ice fishing boots, you should be well aware of your requirements and the usual weather condition/temperatures.