Chadwell is my top choice. I personally believe that the best coaches generally work their way up from lower levels as head coaches. Rather than career assistants who make the jump to the top job. Running a program, hiring/managing a staff, managing a game, having the buck stop with you, is a whole different skillset. Anyone who's never been a HC has to learn those skills, some great coordinators fall completely flat at the big job. Chadwell has won pretty much everywhere he's been. Bill Clark also fits that profile, if he's healthy and wants back in.
Key is probably my second choice, assuming he'd agree to a favorable contract: 4 years, ~$2 million a year, very low buyout, incentives - spend the savings on upgrading the assistants. I've seen enough to think he'd be a steady hand and possibly a very good coach once he settles in. Wouldn't be an exciting hire, but we could do much worse. He's already clearly a better HC than the last guy. I'd rather keep him than go for another 'reach' hire if we can't land a top candidate.
Willie Fritz would be a solid choice, but he'd probably be a short-term stabilizer before he retires. I'd rather keep Key in that scenario. Mike Houston is also interesting, but his results at ECU aren't as inspiring as other options. Tyson Helton looks like a good coach, but I don't feel like he has enough experience in the SE. Huff, Lewis, Wommack and Deion don't have a long enough winning track record, IMHO. Too big of a dice roll for where we're at.
I'd be very happy with O'Brien. But he's being mentioned as a candidate for every open job and may have a return to the NFL as a goal. Not sure he'd take Tech. If Batt/Cabrera can offer him enough resources that he wants the job, then great.