It only proves that saying Imagine is a terrible song is going against the grain. Of course popularism is often wrong, but in this the popular opinion is right, in my opinion.
It's a great song that captured the zeitgeist of a time when the Vietnam war was on the news, religion was beginning to be questioned openly and faulter, the nuclear arms race had led to the CND movement, and the hippy culture was ending....but even though it had to die, the essence of love thy neighbour, be part of a community that cares, peace not war, was an important message.
Musically, ok it's hardly cutting edge in it's chord progressions, but some great little harmonic twists, and the melody is beautiful and very unlike Lennon who preferred to sing a very flat melody, the melody sounds more like it had been written by McCartney to be honest.