The ultimate answer to this question is "how much money is made each day." But the immediate answer is: price. Price is the most important. Price is the most real. Everything a trader knows about the market is reflected in price. Everything the market knows about itself is also reflected in price. Price accounts for volume, open interest, and everything else.  

Price is the only thing we need to observe because it is the indicator of what the market is doing. All fundamentals are contained within price.  

Supply and demand are also embedded in price. Price tells the whole story.  

Sloman said:  

"Everything people say, when you look deeply, boils down to one thing: price is the only thing that matters. When we observe the market, nothing else is important because everything is reflected, summarized, and condensed into that one thing. Everything."  

Price is the most real.