The most popular seems to be Nakhla double apple and Al Fakher grape. BUT! Some people don't like the anise in double apple, and AF has been notorious to have bad batches of "ketchup-like" tubs of grape shisha with finer cuts of tobacco. If you're smoking Nakhla DA now that's one of my staple shishas to always have in stock, for every box of it i've bought over numerous months it's been a nice consistent smoke and flavor.
As far as the best though, well of course that's all up to your personal preference and that's where the review section of HP really really comes in handy. Before i ever try a new flavor i always check the review section and it's helped me out greatly in finding good flavors and staying away from the bad ones. I say be brave and try all sorts of different brands and flavors and you just might find something less popular you're crazy about... and be sure to let us know or put up a review!