Capture One Pro here. Excellent workflow and even better IQ. I've said many times here the difference in IQ between CO and DPP is larger than kit lens to L. Amazing results in very little time. One of the acid tests for me is rendering 24K gold properly and CO nails it immediately, DPP makes it look like brass no matter how many hours I spend messing with it and color profiles.
Full disclosure: My thought when doing evals was the software from the company that has a medium format camera and lens lineup was probably the place to start. So I did and was impressed enough to buy and never bothered testing others (other than DPP). I don't have much time to fiddle with my hobby so I wanted a software aimed at the time=$ crowd and recognized it immediately in CO.
That said, I am also an Adobe Premier Pro (video) user and swear by it even though I'm not a fan of the quirky UI and workflow. Adobe would have been next on the eval list, but after trying CO my thoughts were Adobe might equal CO in IQ but not surpass it and no way the workflow would be as good based on my experiences with Premier Pro, so no reason to go there.
No opinion on the others since I have no experience with them (except that DPP is horrendous). But given their respective successes in the market it's probably safe to say they are all good products. So hopefully the value of this thread is for those either shooting JPG or using DPP and looking for ideas how to raise the bar. I doubt seriously there is compelling reason to change from one "good" RAW processor to another.
 If you don't have color calibration for your monitors, I'd say there is much more value dealing with that first and then moving into improved RAW processing. As much as CO improved my setup, it would be a virtual waste without first getting the monitors calibrated.