To me the MOST INTERESTING at the present time must be Andrew because of his silly coverage of his personal going-ons. Most people in USA [unless part of a royal internet forum] could not name or care about anyone but the British Royals [shame but true]. Personally I favor Letizia and Mary, but enjoy reading about them all.
__________________ Forgiveness is the fragrance the violet shed on the heel that crushed it - Mark Twain
I think it depends on what do you mean for "interesting". For gossip, scandals, rumours and new events: Charlene of Monaco, Sofia Hellqvist (not royal yet) and maybe Madeleine. Kate is not a novelty anymore I think...
"Yet, walking free upon her own estate
Still,in her solitude, she is the Queen".