Then the marketers/sales guys. They will be similar to the users, tell you what features are missing that cost them sales. The features that let them close deals.
Add in your help desk. Where are the complaints? Bugs? Things that always break or confuse people.
Then let management review that and prioritize. Weed out the crap.
Then hand all that over to the UI/UX team and let them wireframe a set of fixes. Let competent management and some specialists review the recommendations to see if it seems like good ideas.
Last it goes to the engineers. But they get input on what will really be implemented based on time and complexity so the impossible requests don't get to the top of the list.
It's a team thing. No one is the "most" important.