definately music first. but if you write the whole song before you think about lyrics they (at least for my band) end up feeling tacked on
Ibanez Talman TC630/Vox VR15
1970's MIJ ET-280/GK Backline 600 300W Head & GK 400W BLX Cab
Most of the time I don't begin writing lyrics until I get some music going first. I have never written lyrics first.
I usually write one verse or so, find music to fit it, then write the rest to match. It usually doesn't work very well though I get two or three meh verses and then I get frustrated and start another song.
usuallt music
sometimes i have just sang out some lyrics then wrote the music a few times but the music then sucked
I tend to write the music first then try to write lyrics around the music.

However, sometimes I have lyrics already written and can write music over the lyrics. It really depends on what type of composer and/or lyricist you are. Some have many lyrics already written and just need to find music to fit them and others have lots of music but no lyrics.

I generally find that writing the lyrics last is a slower process than having them already written.

It's really up to you - there is no universal rule. Whatever works best for you
depends bro...sometimes i'll have a killer idea and start writin lyrics down and then later when im jammin i will come up with a good riff and use that to get the ball rollin to lay out a song.... alot of times i will come up with alot of good ideas for the music when im practicing alone or at lessons. for me its all about what im doing at the time and what ideas come to my head....i always carry(sp?) a small hand heald tape recorder around in my car just incase i get an idea that i know is worth exploring and i dont want to for get it.
Im gonna pistol whip the next guy that says shenanigans !!!!

Sometimes the poetry of the lyrics are the focus of the song. Other times the lyric is secondary to the music.

I find putting music to poetry easier since the underlying rhythm already exists.
^ Quintex has a goood point, changing the rythm in music is a ton easier then changing the lyrics to fit the rythm, I still usually start with music, but I'm trying to learn how to write the lyrics first, just pretty much so I can do it both ways.
Quote by leg end

"Roses are red,
Violets are bitchin'
Goddammit woman,
get back in the kitchen"