Argument for: body of beer, the sugars, the hooch. No malt, no beer.
Argument against: as a matter of flavor, something of a pipsqueak.
Argument for: the mighty spice, rebuffer of infection, bringer of bitterness, flavor, and aroma.
Argument against: you can have a beer without hops, but not without malt or yeast.
Argument for: lagers, ales, and wild things; yeast determines beer's very nature.
Argument against: yeast schmeast; people made plenty beer before they even knew what it was.
The correct answer? Water. Because, you know, yeast, malt, and hops piled up amount to compost. I kid. The actual answer is *. All three are enormously expressive and their influences are easier to see in different beer styles.
Hops are the biggest blowhards, no doubt. They have the greatest capacity to overwhelm a beer--but in many styles they are nearly disposable. Yeast provides the alchemy that makes beer spirituous, and they are the deep-thinking philosophical members of the trio because they do determine a beer's nature. They are not far behind hops in their capacity to make a big impression, either. Malt is the hardest case to make, but the gluten-free movement does an excellent job. Barley malt's presence is subtle and you can easily overlook it, but the parade of sorghum counterfeits, thin, sour, and unbeery put proof to its importance.
It is a debate with no answer, but a fun one to have. Your opinion?