I'm afraid besides "Imagine"(which is one of my all time favourites) I just don't know much of John's solo work.

But "Because" which he wrote with Paul, is for sure, one of my top 5 (so difficult to choose between Beatles songs! hehe). I love it for it's depth and simplicity...

Because the world is round it turns me on
Because the world is round, ah
Because the wind is high it blows my mind
Because the wind is high, ah
Love is old, love is new
Love is all, love is you
Because the sky is blue, it makes me cry
Because the sky is blue, ah
Ah