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At least for me, there is no real perfect answer. I've owned / own pretty much all mirrorless besides the Ricoh system. Some thoughts from me on what I've used, FWIW:
1) Nikon's "1" System - Badass AF and that's about it. Nikon doesn't seem too committed to the system, though, and it seems a bit geared toward casual snappers. Not a lot of real high quality native glass. I wasn't impressed with my V1 rental setup (including some DX and FX glass to test w/adapter)
2) m43 - Oly's got a good thing with its OMD, I really like the swivel LCD, the IBIS is good if you need it, and it has fast AF. m43 lens lineup is also very fleshed out, great setup. I don't like base-ISO IQ as much as some other systems, though, but it's fine for most purposes. I kept my GX1 because it's so damn cheap it's not worth selling - might get the 20mm/1.7 as a generic tiny walk-around camera. IQ of the GX1 is behind the OMD, and both are behind the NEX and Fuji systems.
3) Fuji - So far so good, I like the styling of my XE1 but it can't touch the OMD for camera performance or glass options. What Fuji *does* offer so far in glass, though, is pretty damn nice. I like Fuji's IQ the best of all mirrorless.
4) NEX - I have the 5N and while a decent camera, the menu system and controls of the camera are infuriating, especially when constantly hitting the stupid record movie button accidentally.
Less impressed with the lenses. IQ is very nice with good glass and though I prefer Fuji's color response, NEX is also pretty good.
Adapting lenses - yeah most mirrorless will adapt lenses just fine, but you have to ask yourself what you're gaining - and it's not much. You'll lose the weight associated with the FF body, but the glass is still there, and now you need an adapter to mate it all up -- if you plan on keeping the native glass, you also have to deal with the crop factors induced (1.5x or 2x or 2.7x if you go Nikon), which might ruin your lineup.
Best bet, if you truly want to lighten the load, go native all the way. Oly's lineup is pretty damn complete, though once you get past ~200mm FOV (using the 35-100/2.8), the quality options diminish.
Fuji's got good glass from ~21mmFOV to ~300mm FOV, but we're still awaiting their 56mm/1.2 and whether the 10-14mm zoom is going to be any good (if history serves as a basis, it will be).
You might also really consider going extremely light and getting an RX100 only. I've been strongly debating that with my mountain biking excursions and might give it a rental to see if I like it.
If you're mainly hiking and don't care about blazing AF speed, you might also consider the Canon G1X camera, better IQ than the Sony but bulkier (still small though). RX100 good for a real jeans pocket camera, but maybe not so important if you're hiking already with a small backpack, etc.