I believe it depends on the song you're listening to and the genre it belongs in. For the most part, a good song has a good balance of both a good melody and instrumental and good vocals and lyrics.

Take rap for example, a good rap song usually places a lot of emphasis on lyrical content and vocal delivery all the while not sacrificing the quality of the instrumental or beat being used.
Or rock, a very catchy and often complicated riff as the driving melody, possibly a guitar solo thrown in the mix to jazz things up a bit, at the same time lyrical quality is still maintained.