I'll try to respond as objectively as I can. R2F is right, desirability is certainly in the eyes of the beholder.
Comair is well-liked because they have the "gold standard" contract for the regional industry. Of course, they had to strike to get it. Down side is they only have one pilot base ... so you'd better like CVG or be happy commuting there.
CHQ is well-liked because they are growing and have relatively short upgrade times, as well as the relative stability of having their airplanes spread over four "Big Six" partners. (If one fails, the whole company doesn't go down the drain.) Down side for some might be that they have a lot of "outstation" bases that aren't necessarily commuter friendly.
People dislike Mesa for a few reasons. Their contract is somewhat behind some other regionals in certain areas. Their management (particularly the CEO, Jonathan Ornstein) is roundly despised by pilots everywhere.
The biggest complaint I hear about Pinnacle is that they pay you nothing until you complete training ... no guarantee, no per diem, no meal allowance, no hotel, nothing. Some people also don't like their domiciles ... unless you love six months of winter, PCL won't be a good place for you. (Memphis runs fairly senior and for some reason some people don't like the city ... personally I think it is a great town but that's my opinion.)
Again, desirability is in the eye of the beholder. Maybe it is because you know someone on the inside. Maybe it is because Airline X has a base in your home town and you'd prefer not to commute. Maybe Airline Z only flies east coast and you're a lifelong Californian. Maybe some people don't want to work for Chautauqua because they can't spell or pronounce the company name. I don't know. Each person must make their own decision about where they want to work ... what might be great for me could be quite poor for you.
Clear as mud, right?