Mother Nature's Son is Paul's most whimsical song, in my opinion. But out of this list, I'd have to say Paperback Writer. Do you suppose Paul just woke up one day and said "Hmm, think I'll write a song about an aspiring novelist and then have some tea."
I've discussed this on other threads, but Martha My Dear is deeper than a lot of people think. I always found it to be rather sad, as I interpreted it as being a bittersweet tune about the end of a relationship.
