I think we have discussed enough who is better MD or who is a great singer. But I think for a hit song lyric and tune are like really important. Even we can further go ahead discussing incidents when song was flop because of singer even though both lyric and tune are good.
Different people look for different things in a song.
So tune or lyrics is a personal question. However, I feel that, for a vast majority of people, it is TUNE that is most important. Lyrics are typically forgotten, although it is true that it adds value to the tune. A good tune without good lyrics can easily survive in people's minds, but the other way is not true. So my vote goes for TUNE .