Have to start by agreeing, there is no one camera/format that is best for all. Anything all-around would be a compromise.
With the introduction of Sony's A7r II things are getting interesting. I have and use one, read several reports/reviews and its impressive.
The weak point of previous models from Sony (except A7 II) has been slow to poor AF. No more, it is a good performer. For normal use,
it has been excellent. Will be trying with more challenging subjects, probably birds in flight. Have a feeling it will do great with my limited experience
doing pigeons going by my balcony. Used a 70-400 Sony G II.
The sensor seems to be the best on the market at this time for a full frame camera (35mm equivalent)
Tough decisions if you are firmly with DSLR and never tried something smaller. In my case, also having a Leica-M system along with a DSLR, has been a no brainer
switching to Sony. Might even replace theDSLR? On paper, it is lacking the burst speed for action, currently 5 fps. On my one trip for action (puffins), took a Sony
A77 II with excellent results.
If you are able to see some of the video reviews, pay attention to the AF modes. Having the ability to use continuous servo and keep one spot on a subject while it moves or
you move the camera is impressive.
Interested to see if there have been any members changing their minds regarding mirrorless systems, particularly Sony.
If you are even tempted to try one, strongly suggest renting one before buying. Also becoming totally familiar with its functions. Can even rent an adapter for your current lenses
and check the AF performance. I have only used it with one Canon Lens, the 24 tilt/shift but the lens is manual focus. For this one all electronic connections work which is nice compared to all others used before.
al