The Voice

First the singer. Or perhaps last - chosen to fit in with the style of music setup by the other musicians. But I think that in its formative years, the lead singer clearly defines the nature of the band, and even if they eventually leave they will have left their mark on the other band members. Just ask Audioslave who used to be Rage Against the Machine before Zach de la Rocha left - and now look at how their music has changed from working with Chris Cornell instead.

So I say choose the singer first. It must be someone with a distinctive voice capable of expressing exactly the emotions you want expressed. You don't want someone who simply can sing well, or has a good voice. You want someone who can accent that singing with soaring heart-wrenching high notes, husky sultry staccato, deep angry growls, or wailing screams. You want someone who will sing his songs in a way that no one else can quite re-create - who goes beyond the notes and captures something no one else saw in the music until he showed it to them. This is actually the kind of thing that you want from each musician but especially from the singer. You also definitely want the singer to write his own lyrics and melodies. You want someone who has not just a single cause that they are always on about but a lot of passion for a variety of topics - someone who can express their worldview from many angles with great emphasis. For example, you don't want Zach de la Rocha who left the band because he couldn't think about anything but leftist extremist politics and that's what all their songs were about - though they were good, it wasn't meant to last and I think it limited the band. You want someone like a Chris Cornell or James Hetfield or Robert Plant or Roger Daltry.