the 20th century ran from 1st jan 1901 to the end of the year 2000 btw.

and bohemian rhapsody and tomorrow never knows might be good tracks, but not that great songs. how often do you hear someone singing either of those songs, and getting all the words right?
a good song is one that is usually easily sang and covered, such as "happy birthday to you" which was from the previous century

so a good example would be imagine by john lennon. good lyrics, easy to sing and it's a classic. not sure if it's the best, but many consider it to be