The gold. It is volatile and there are always plenty of trading opportunities during the day.
So, yeah, I think that gold is the most profitable instrument to trade.

There is no such thing as more profitable asset or less profitable one. There are only those for which you have a honed trading strategy. But, there are more liquid markets and less volatile ones.
This is the reason why people initially commence with forex trading, it's a more intuitive market for traders with little to zero knowledge. Volatility is meager, when it comes to trading off the market's peak hours.
On the contrary, some traders prefer volatile markets because of sharp price jumps which allow to earn on them. I'd advise forex...