Which brand of dental crowns looks natural and have good life span (40+ years if proper care is taken)?

It's not the brand: It's the experience and expertise of the dentist and the laboratory technician, along with the proper case selection, execution and home care maintenance\oral hygiene that determine both the esthetics and longevity of dental crowns. The actual materials has very little to do with it. All ceramic crowns look the best. Btw 40+ years is unrealistic! 10 or more is more likely although 20 is possible.

Nonsense: There uis no "brand" of dental crown. They are all made for the individual. There are many brands of dental materials. A good lab tech familiar with whatever is good in his hands will make something that, with proper home care by you, will work. The porcelain and metal could last for thousands of years.

Crowns: Many of the all ceramic crowns can have a natural appearance. A crown can last for forty years, but the appearance of any natural tooth adjacent to will change over time. From wear staining and just the natural aging process of the tooth. So if you take meticulous care of the crown, any crown, when done properly it may last forty years. But you will probably notice it years before that.

Ceramic crowns.: Metal-free, bonded crowns look the best. The closer that ceramic is to enamel in wear resistance and in coefficient of expansion and contraction, the more stable the bond will be as well. The best in my opinion are produced from cad/cam technology called cerec in the dental office.

None: I have not read a clinical reseach demonstrating clinical success of 40 plus years. However, research states highly variable demonstrating percentage failure within 3 years to 23 years of use. The answer is a case to case basis, which depends of various factors, including skill of dentist, lab fabrication, type of porcelain crown, and patient variables - diet, parafunctional habits, hygiene.

Studies: There are no studies/ research that can give you a definitive answer for any type of crown that is the magic answer for lasting 40+ years, ... You may have crowns that lasted over 40 years, but every mouth, every tooth, every bite, every diet, every bacterial load, every oral hygiene/homecare, every position, every load, every brushing/flossing angles is different from tooth to tooth, person to per.