I think it's very individual. And from my experience it's still currencies hands down.
Forex pairs brought me a lot of profits and I think will bring me even more.

ActivTrades has an excellent range of tradable instruments from all the possible asset classes. I think for every trader out there it is possible to find his preferred trading instruments with this broker.
As for the profitability, that depends more on your trading strategy. If you haven't found yet your favourite instruments for trading, I suggest you focus on this thing, explore, and trade different instruments from various asset classes and see which one aligns with your trading style.
If you like more dynamics in your trading, you should choose more volatile instruments, such as major forex pairs, gold, or maybe cryptocurrencies.