What is the best type of flux?

In the realm of welding and metalworking, the choice of flux is crucial for achieving optimal results. Could you elaborate on what factors should be considered when determining the "best" type of flux? Is it primarily the chemical composition, its ability to remove impurities, or its impact on the weld's mechanical properties? Does the application - whether it's for steel, stainless steel, or another alloy - significantly influence the selection? Moreover, are there any emerging flux technologies or advancements that promise to revolutionize the welding process? Understanding the nuances of flux selection would greatly aid in making an informed decision.