The best bio-compatible dental casting alloy is:
TITANIUM AND TITANIUM ALLOYS
In vitro evaluation of titanium biocompatibility, percentage attachment efficiency, and proliferation of human fetal fibroblasts and human gingival fibroblasts reveals that a surface layer of titanium oxide (Ti 2 O 3 ) has the ability to coexist with living tissues and organisms. Based on these studies one can conclude that titanium is relatively nontoxic, noninjurious, and not physiologically reactive. Titanium has a light weight, high  strength, and excellent durability when exposed to chemical agents. However, it is susceptible to attack by acidic fluoride products.
Phillips 12 ed page 126
Co-Cr
Ni-Cr
Titanium alloy
Gold