For me, a song that has a strong, original, and pleasant melody is a good song. Some songs have strong melodies, but they’re annoying. “Pleasant” is subjective, but what some people disparagingly call a “sugary” or “sweet” melody is a pleasant melody to me, and therefore a good song. Second, having good, meaningful lyrics is important too. But melody is the most important. Also, a song that drags on and on, or lacks a good beat and rhythm, is not a good song to me.
These are good songs——
http://www.youtube.com/watch?v=JMls9gkSqg8
http://www.youtube.com/watch?v=eki_PRzNk-Y
http://www.youtube.com/watch?v=N5f0mWnM9gU