I definitely prefer Smoke On The Water myself. I'm not comfortable with the lyrics to Imagine, which I must admit I tend to see as Marxist. I sometimes think that people see Imagine as a song about people living in harmony, whereas I hear it is a more political song. By contrast I really like how Smoke On The Water tells the amusing story of Deep Purple's attempts to record the album. Of course, there's also that guitar riff - it may be overplayed (the riff even more so than the song) but I love it. I also love Jon Lord's organ solo at the end.