I've been shooting with a Nikon D3 for the last 7.5 years. Without exception, it's brutally fast and accurate, always.
But lately (perhaps because I've been travelling), I've been craving a smaller/lighter body. The D3 is damn big and bulky. So now I'm strongly considering getting a new camera.
I'm a Nikon boy though, and all my lenses are Nikon, so that's the way I'll go. If I do buy a new camera, it'll be either the Df, D610, D750 or D810.
My preferences are for speed and high ISO. Pixel count is totally unimportant to me.
Have a look at the specs for these for cameras. If you wanted to buy one of them, tell me which you would want, and why.