Up until recently sports oriented bodies (1D and 7D on Canon side) had always been crop sensor. The crop sensors having smaller mirrors allowing for faster frame rates. Crop sensors also give more "pixels on target" aka "reach", something that is essential to certain genres of photography like sports and wildlife. Let's face it, the cost between a 300mm lens and a 500mm lens is not insignificant.
Meanwhile Canon always separated their full frame pro bodies as being geared towards studio work (hence the S on the 1Ds line) and portraits. Genres of photography that benefit more from outright IQ than speed, as well as the DOF difference you get using longer lenses on full frame cameras. Canons release of the 1Dx was accompanies by a statement from their top brass that they believed a full frame 35mm camera to be the optimal, and had been their target all along.

There were an awful lot of full frame Canon 1DxIIs at the Olympics. Which, if I recall correctly, included everyone shooting for Getty.

Personally, I think that when you're talking about outright IQ for the sake of IQ, there are too many variables to assume a full frame 35mm was the end all in the 135 format family. Figure that for several years recently Sony APS-C sensors were outperforming Canon full 135 sensors in outright sensor performance metrics. Add some top flight glass and you suddenly have an APS-C camera with better sheer IQ than a more expensive 35mm camera. Of course Nikon use Sony sensors and manage to get better performance out of them than Sony does (just compare the D810 to the Sony A7R). If you compare the Sony APS-C offering to their own 35mm offering however, you see that in raw performance metrics the full frame sensor does better.
All of that aside, I'd say that any modern SLR from any of the main manufacturers (Canon, Nikon, Pentax, Sony) should have no problem turning out top quality work. So long as the photographer is capable of envisioning it. We've long since surpassed the time where the gear was a major limiting factor. Now it's a combination of personal preference, budget, and G.A.S.
