Personally, I wouldn't buy any, because:
- they pay no interest
- generally not a hedge against inflation
- storage costs
- opportunity cost (could be invested in something that appreciates)
Given the listed options, gold, because:
- lowest premium (buy/sell spread)
- easy to buy/sell
- capital gains tax free in many countries
- portability (high value density per size/weight)
Gold also has high sentimental value, particularly between older generations. They simply HODL it, to pass it on to their heirs*. Other metals are more tied to industrial supply/demand needs, and I think in this case its better to have exposure trough equities in manufacturing.
*What do new generations think of gold? Will they sell it after inheriting it? Will this push the price lower?
You pretty much covered it all. Could add one bullet that diamonds should see a steady decline in years to come as the newer generations are more educated on the true value/invalue of diamonds.