Personally; I'd look towards Silver.
Right now silver is in a bit of a strange spot strike price wise. It has strayed from gold and the USD in terms of it's long; as in very long, established trading history, and is now in my opinion highly undervalued.
That said; I think it's a long term play, buy and hold.
But Silver is at it's low. Currently it costs more than the spot price on an OZ of Silver to Mine the OZ than what it sells for. So mining new silver is not a money maker; a big problem for mining companies.
On the flip side Silver is a commodity that will continue to be in demand; it's used in massive quantities in the electronic industry; health industry; and countless others. It's not just coins and jewelry.
But to be clear: Bitcoin of course would be the best investment.
BTCBTCBTCBut if I did add more precious metals to my investment portfolio; whether by buying physical bullion or trading commodities; I'd buy Silver right now over Gold.