While Imagine is always going to be the standard bearer of Lennons legacy, i really love Instant Karma and Cold Turkey. They both rock and if it wasnt for the primal scream therapy at the end of Cold Turkey, it would of been regarded as a Beatle great. I'm still not sure why they vetoed it has Paul or Ringo ever said why.
Although i dont think it was ever released as a single my favourite track fom John is Oh Yoko (John and Paul should have got together and released Let Me Roll It and Oh Yoko as a double a-side to show they were still influenced by each other.)