hmmm...
If you take the IRB rankings as a benchmark, that would suggest:
Wales
Ireland
France
England
Scotland
Italy
But before the World Cup, they were:
Ireland
England
Wales
France
Scotland
Italy
So in the last tournament:
Ireland & England dropped [lots!]
Wales & France more or less stayed the same: Wales at a strong level, France still, um, a little confused ;-)
Scotland & Italy closed the gap - with Italy still to appoint a coach for 2016.
So, in 2016?
England are rebuilding - the power's always been there - the challenge is to find traction to use it. Eddie Jones will, but maybe not consistently until 2017.
The Scots... well, if you can take Australia to the wire, and learn how to close out games, then anything's possible. Though probably only on a game-at-a-time basis: it's difficult to see the strength in depth to mount a sustained challenge, without slip ups.
Ireland have lost two giants in Brian O'Driscoll & Paul O'Connel - will it make a difference?
France will remain confused, most of the time, and mighty dangerous for 15 minutes every match.
I end up looking at Wales as the ones to beat - and pretty sure any of the other teams, on their day, can beat Wales [well, maybe not Italy]... but not across the whole tournament.
underpants used to play play it years ago . 
