In my lifetime ...
1. Cutcliffe
2. Walt Harris
3. Fulmer
4. Jim Chaney
5. Randy Sanders
6. Mike DeBord
Cutcliffe was the best I've seen. I know some are arguing that Cutcliffe had the best players to work with, but they wouldn't have necessarily been "the best players" without him. The obvious comparison is Erik Ainge pre-Cut versus Erik Ainge post-Cut. Ainge was a totally different player after Cutcliffe came back. Cutcliffe made everyone look better and he continues to do so at Duke today!
Walt Harris deserves more credit as many have mentioned. He's arguably in contention for #1 on the list.
Chaney is underrated primarily because Dooley sucked so bad, but Chaney's long-term track record has been very good, and our offense certainly was not the primary problem in 2012 (defense, weak recruiting, and Dooley were the problem!)
DeBord is easily the most overrated OC we've had in the past three decades. The argument for DeBord is "the stats", but the stats were inflated by plays per game / fast-paced offense. DeBord's teams had a very high Turnover-to-TD ratio and the 2016 team was extraordinarily undisciplined (very high number of penalties). The 2016 team also underperformed significantly against some stronger defenses (Bama, VT, Vandy, SC). DeBord was better than expected when hired, but not nearly as good as VN seems to believe right now. He certainly deserves credit for destroying Florida's defense in 2016, but I think people are giving him too much credit based on a moderately successful output from one of the most talented offensive rosters we've ever had. If DeBord had been OC in 2013, 2014, 2005, or 2008, VN would likely be cursing him.
Randy Sanders struggled in Knoxville, especially post 2001, but he's been very successful in other stops at Kentucky and Florida State. It seems as if Fulmer was holding him back substantially here and when you go back and look at Sanders' offenses, he did have good success at times, even if we mostly seem to remember the 2002 and 2005 debacles. So Sanders is probably the most underrated of the group. If Sanders had only been here two seasons (1998 and 1999) like DeBord, many would be saying he was #1 on the list.
Coach Bajakian is a difficult OC to evaluate primarily because he had so little to work with. No OC, no matter how brilliant, was going to have big success with that horrendous O-line in 2014 or the complete lack of skill players in 2013.
Dave Clawson is an example of a "good OC hired at the wrong time." You look at his track record, he's succeeded everywhere he's been, but it's often taken 3 - 4 years for his offensive system to take root. That said, if we're just going by his time at Tennessee, he was the worst of the bunch.