For front teeth, best cosmetic result achieved with Procera/Lava/Impress crowns but they are case specific. The advantage of the first two is that the substructure which provides the strength necessary for front teeth is made from a white material called zirconia or alumina thus allowing light to pass through crowns for a more natural appearence. They are the top of the range crowns and the laboratory fees are higher than other crowns because the plaster models have to be scanned (usually in an engineering company the lab uses) and the substructures milled by the crown companies (nobel biocare or 3m), before being sent back to the Lab to have porcelain superstruture placed on them. In some cases I ask patients to visit lab for delicate shade charactaristics but most of my cosmetic work is done by top labs in London and California so it's not practical to visit. Most dentists doing the top end stuff will take close up photos of your teeth from a number of angles and email these to the Lab.
Expect to pay more for these crowns but you will only do this once, also Procera & lava come with gaurantees from the companies who make substructure, you get a little card from them at fit appointment with a serial number and usually the shade.