This is a trick question.
The users drive the software; their use defines it and their input is the most important.
However, the users are their own worst enemy. They have trouble translating the effects they want into the designs that are required in the software.
Thus I think the users need the most input, but that needs to be filtered through developers, the smart managers (1-2% on any job), and the user experience folks.
That is, if you want functional software. Users are their own worst enemy and are self-defeating; if you just want to make some bucks quickly, the marketers are the most important as they'll go flatter the users and come back with a list of buzz-words and then make sure the software has those features, even if in barely functional form. But you'll sell more product in the first cycle.