Initially, the most important part of a song for the songwriter is the part that makes you realize you are writing something meaningful, maybe even something wonderful. Finally, for the songwriter the most important part is the last few edits when you know you have fulfilled the inspiration that got you this far.
At the record store, on your web site, on the radio, on mtv, and any time anyone is referring to the song, the most important part of the song is the title.
When you are singing along, the most important part is whatever helps you remember the words of the song -- the title, the chorus, the melody, the story. When you are listening, the most important part is the part that makes you want to hear that song again --- the title, the chorus, the melody, the story.
When you are looking back to younger days, the most important part of the song is the part that represents how you felt when the song was popular -- the magical part that turns that special song into a time machine, transporting you back to a precious time of life.
For me as a writer, the most important part of songwriting is the feeling of joy that overcomes me when the words and music I am working with start sounding like a song.
Keep writing,
Don