HI there. A pretty good stable biocompatible material is apparently Cerec porcleain.

But if you cannot get that? Then I'd probably opt for a material that has been found to be safe for the majority of people. Cerec seems to be one of those, but it is very expensive. However, it also lasts most of your life, or a lifetime. It is as good as gold (literally) I hear.

I would recommend you read this link through:
http://www.drheise.com/dental.htm
And scroll down to where it says
Dental Materials: The Best Choices
Recommended Biocompatible Dental Materials

This should give you a good idea of what materials have been found to be the best.

Though I would not entirely agree with it in regards to composites, as these stand up pretty darned well in many people, though some may have issues. There will be people sensitive to almost any material too, so it is not an easy decision to make.

Whatever material you decide to go with, if you do go for cerec, you need to go to a cerec dentist who is experienced in replacing it! Because it takes skill and experience.