Congrats on the 6D! Each RAW file has an embedded copy of the JPG, and I'm fairly sure that's what ZoomBrowser uses. I can scan through images insanely fast with ZB. The embedded JPG will have all the white balance, picture styles, noise reduction, etc all applied, so it will look pretty good.

ACR (Adobe Camera Raw) doesn't use the camera settings and will have it's own ideas on white balance, color tones, and how to handle noise. I find ACR does a better job with noise than DPP (and thus better than ZoomBrower's JPG), though I haven't tried the latest DPP... or the latest ACR for that matter. Sometimes I find ACR's interpretations of colors questionable, but for the most part ACR is good.

I have no idea what OnOne shows you, but it also will convert the image in it's own way. Perhaps by default it has no noise reduction enabled at all. The OnOne is probably being more honest with what your camera actually captured, and is intended to be a specialty tool to tweak the conversion process to your liking. ZoomBrowser and ACR on the other hand, have default 'make it pretty' settings applied, which can be tweaked. Perhaps OnOne is aiming for a speedy preview (like Corel's AfterShot Pro), and will look better after full conversion. Perhaps noise is it's downfall (like Corel's AfterShot Pro).

Take a few days to play with it and let us know how it goes.