Edvald Boasson Hagen.
The dude has the broadest overall skill-set of any (presently known) rider who will still be around come 2019. With a couple more years of seasoning and experience I don't think that there is a classic he couldn't potentially win.
Cav is an intriguing prospect. He's obviously got the best sprint of anyone out there right now, but his preference clearly goes out to sprinting in stage races. He could perhaps develop the qualities, like a Boonen or Hushovd, to compete in some of the Flanders' classics (something his Scheldeprijs victories clearly show he has the potential to do) but I imagine he'll prefer to stick with his Giro-Tour strategy for now, which would conflict with being in top-form come the first weeks of April.
That said, if he goes on to make Milan-Sanremo his personal b!tch for the next decade, picks up a flat-ish World Championship once or twice, and claims a few smaller classics (like Paris-Tours, Paris-Brussels or the Hamburg Classic) he'll definitely merit serious consideration 10 years from now,