Like fryetrading said - Commodities are the king of volatility. I don't like to play favorites since I trade them all.
you could argue "penny stocks" and "bitcoin" are potential instruments, but those are too illiquid imo
You ask for the "Best Trading Instrument" in the title, and "the most profitable" in the post...the best and most profitable are not necessarily easily matching or congruent. I only know options. So I'm bias and partial to options. that's my answer. The "Best" thing to trade is obviously very highly subjective from person to person; The "most profitable" is much more closer to being a linear answer, Options are leveraged products...it doesn't take that much money to have potentially great rewards. (but like everything in life...the devil is in the details) Everything stems on your ability to predict the market. Only this matters. The Future...predict it; whether that be 1 minute, 1 hour, 1 day, 1 week, 1 month, 1 year from now, If you ask me, it's easier to predict and/or manage...things on an intraday time frame, Beyond this, the future is essentially just a big unknown shit storm with way too many variables beyond your control, to foresee and grasp, Controlling or managing variables...is key to winning in the market, If you don't got this...you're just a fool. with his money, to be soon parted. Reasonable expectations unfolding and happening -- not simply gambling blindly, ...if you want to treat the market like a casino...then you will get casino returns -- which is leaving with an empty wallet.
There isn't a single, obejctive "best instrument", nor is there a single, objective "most profitable instrument": it all depends on your own skill-set and experience. Sorry if it sounds like a cop-out answer, or an unhelpful one, but neither of those observations actually detracts from it being true.
It's the one you have a strategy and experience with. For me it's stocks and I'd be a bit of a fool trying to jump into the others without a lot of testing which I just don't have the time for.
If I had to choose only 1 instrument to trade than hands down for me it is the NQ. It trends very well, responds great to classic chart patterns, is less expensive to trade than most - it is a breakout market with minimal back filling.
Best Trading Instrument The best trading instrument is between your ears. Use it to trade successfully based on that well guarded secret that has been handed down from father to son for centuries, being: Buy from those whose trading ability is less than yours, and later when those same traders come to buy, let them have your shares.