The QMJHL seems to encourage offensive creativity and, as a result, it seems we get some excellent high end offensive talent out of that league. Crosby, Bossy, Lemieux, Giroux.
The OHL has the largest concentration of hockey players to draw from so you see plenty of top end talent coming from that league. Gretzky, Spezza, Tavares, Stamkos.
The WHL tends to be a more defensive/physically oriented league and you end up seeing a lot of good defensemen coming out of the West. Keith, Weber, Phaneuf, Niedermayer.
As far a depth and overall talent goes, I believe it is OHL>WHL>QMJHL