If this seems like an odd question to ask and the obvious answer should have been "both!", please allow me to explain by giving you my answer to the above question.
The first thing that strikes me about a song is its tune, or melody. Lyrics are secondary to my ears.
I never really gave much thought to this at first, until I realise that I could listen to a foreign song which is in a language I do not understand (Korean, Japanese, French for example) and find something beautiful about it in spite of my incomprehension to the meaning of the lyrics. If this proves anything is that the foreign singer could be singing something as unexpected or unrelated (like his pet crocodile or sumfink) to what the tune seemed to be expressing and I would not have known it, and still enjoyed the song for its melody and nothing more.
In saying this, what if its an English or Malay/Indonesian song? Would I be influenced by the lyrical content of the song if the tune was something very beautiful to me since I actually understand these languages? I think the answer is, no I would not be affected by the lyrical content either. Truth be told, some songwriters are better than others, and some music makers are better than others too. If the best of both come together to work on a song, you can bet the result would be a chart-topping hit. However, there are those that are not as good but somehow a song or two gets produced anyhow (and the results are somewhat mediocre or lacking).
But that's besides the point.
For me, it'd be great if the song had wonderful lyric composition as well as a great tune to it. However, if the lyrics are mediocre at best, but the tune was hands-down awesome, it'd still be a great song to me. Therefore, in my own opinion of music, what lays the foundation to a great song is not the lyrics but its melody. The song can be ruined by disastrous words for lyrics fosho, but of course it'll take some grand disaster of a songwriter to destroy a strong foundation of a song which is its grand melody and tune. Hurt both and you get a really, really bad song -- totally not worth listening to.