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A lot of this will be personal preference, so it's worthwhile to try them out. For instance, I really don't like the NEX cameras. They just rub me the wrong way. They feel terrible in my hand (the grips are way too cramped up next to the lens for how I hold the camera), and it feels like operating a computer rather than a camera. Nothing wrong with computers...I love them and have built them for over a decade...but I don't want my cameras to feel that way. The NEX system also has pretty mediocre lens offerings. The size of the lineup is growing, but even their really good lenses would only be 'Ok' in some of the other mirrorless systems. Other people love them, and if you're shooting exclusively adapted manual focus lenses, it may be the best bet.
I think Micro 4/3 is definitely the most mature of the mirrorless systems. Bodies are responsive, with blazing fast autofocus (single shot) and image quality has drastically improved with the latest round of sensors, and are now on par with many APS-C cameras (e.g., most Canon APS-C DSLRs). The in-body stabilization of the E-M5 is phenomenal. The lens lineup is huge, and while there are a lot of consumer lenses in that lineup, the high end is also there, with many truly outstanding lenses from ultrawide to telephoto.
I quite enjoy shooting the Fuji system as well. The cameras have controls very well laid out, and it's enjoyable to shoot a camera that is simply built to be a camera. Quick, easy access to real dials that control all the important things, and easy access to needed features. It's really a joy to shoot with. The lenses are also outstanding, as they've catered to the high end shooter from the beginning. I'm using a 14/35/60 kit now, and they're all fantastic...I really love the way the Fuji lenses draw. That said, it has by far the slowest autofocus of the mirrorless options out there (save for maybe the EOS M), and is definitely more useful as a deliberate camera. I really like the image quality out of the X-E1's sensor. In very rare cases, yes, it can produce some odd artifacts with high frequency detail, but I don't find it happens all that much to be honest, and it only is visible at 100% on screen. Prints are gorgeous, files are very clean and have great tonal depth and dynamic range. I think the Fuji system has a lot of real promise.
So, those are my thoughts. I'd avoid EOS M altogether. Canon put it together as an afterthought, and there's nothing it does better than any of the other systems, aside from take EOS lenses natively....but if you're using huge SLR lenses, why are you going mirrorless? Overall, I think m4/3 is the best bet at the moment, and I personally prefer the Olympus OM-D E-M5 as my best overall mirrorless camera, though I shoot a lot with the X-E1 as well.