Dentists are often asked the question, "What type of filling would you want in your mouth?" or "What's the best filling I can get?"
There are a lot of variables to take into account when considering these questions:
Is it in the smile zone?
What is your budget?
What is your daily oral hygiene routine?

In general, if it's not an aesthetic concern, the best material is also the oldest material. Gold.
A well done gold restoration can last significantly longer than any other restoration we have available.
A gold casting conserves as much tooth structure as possible.
Specific alloys of gold are used that expand and contract at the same rate as your natural tooth structure thus reducing the chance of sensitivity and/or fracture.
The alloys used also wear at the same rate as your teeth, so you don't have to worry about a gold restoration wearing down the opposing tooth.
Gold will not stain or discolour the tooth.

What are the disadvantages of gold restorations?
It's more expensive. This one is sort of true. While they cost more initially than a filling, they need to be replaced far less often resulting in less overall cost.
They take more time. A gold restoration takes more time to do well than a standard filling. Normally it takes one appointment to prepare the tooth and a separate one to insert the restoration. Again, since they last longer, this is only true when compared to the initial filling.
It's not tooth coloured. This is primarily a concern with front teeth or teeth in the aesthetic zone.

Gold: It's still the gold standard.