Which is the most effective and most "natural looking" dental crowns material?

Many choices. The quality of the lab your dentist uses really dictates the quality of the finished product. If you are doing front teeth i would have a custom shade done at the lab, most technicians will spend 15-20 minutes with you discussing your expectations. This may cost you extra but you will get to explain your expectations to the guy actually making your crowns, they can pick the best product for you!
Porcelain. There are several types of porcelain crowns that a dental ceramist can use that can look exactly like a natural tooth. An excellent looking crown may cost a little more, but it is worth it to have a "natural looking" crown.