Just 1, you hit it right on the head, for me.
How many songs out there, that you've known for even years that if you were quizzed on, you wouldn't know for sure what the song is even about? Which answers the question which is more important (if there has to be one) the lyrics or the melody?
Take James Blunt's 'You're Beautiful' for example. I just love the melody, but truthfully, any one of the songwriters on this website could create better lyrics than that... I know I could. But guess what? That song is on my iPod ( don't tell anyone,lol). because it sounds pretty.
However, to me tune and lyrics when I'm writing a song are equally important.
Anyway, back to the topic...
When I first started writing songs I didn't play an instrument. I would write essentially poems in song structure. That was very good for me because I didn't have a pretty tune or a catchy beat or anything else getting in the way, so all of my efforts went into the lyric. That was good education for me at the time (despite the silly teeny bopper subjects... I was a sensitive young man, lol!)however, when I had picked up the guitar and started writing with it, I found I had one hell of a time putting my words to new music. That's when I learned what a metronome was, lol. The worst thing for me was to sacrifice some of my 'baby' for the sake of the tune. All of those songs today are just poems, or well thought-out ideas for songs.
Almost every time I write a song, it starts with a tune. It'll usually pop into my head. Since I'm usually at work, I'll try to find a private place and hum a few lines into my microphone on my iphone so I don't lose it. Most times I will sing the song with words I've pulled out of nowhere( I call them filler words). That tells me how many syllables I need in each line if i go tgat far, and I basically have my song structure layed out... All ahead of time using nonsensical words. Then when I have my guitar, I find out what key I'm in, and play usually just three or four chords over and over and over and sometimes over and over and over again. The music will then tell me what the song is will be.
Other times I'll get a really unique idea and build the lyrics and the melody together.
I won't ever write lyrics first (anymore). It's either tune first, or both together for me. It helps the song evolve better I think, if you can work them both together.