I'm choosing to define "best" as "looks good while still fulfilling the original purpose of coins - to be spent as money".
For circulation coins, gold's too soft, and copper and silver discolour. Aluminium "feels cheap and plasticky" and corrodes easily, while platinum is way too expensive.
Conclusion: you can't beat pure nickel. Hardwearing and corrosion-resistant, yet not so tough that it's impossible to make a coin with high relief.