My bias says Thibs. But I am old school, in the sense where I like when coaches win, in a scenario where they prove to be the most improved team with little/no argument that suggests their success is due to an added player.
But this year there is no clear cut "most improved" type of team. Last year, it was Collins over Thibs for team improvement, but Thibs is a better coach top to bottom. Bias aside, I have a hard time saying anyone is doing better than Thibs this year, and he's going to get HUGE props for helping us weather the storm of injuries.
Corbin deserves a LOT of credit for Utah managing everything that has happened with that franchise and bring a competitor every night. If the Jazz can make a run for 6th seed, then he is in the running. But I think if Corbin were to win, that would be a HUGE slap in the face of Sloan, so I doubt he'll get it in a weak season just because of loyalty reasons.
My rankings right now.
#1 Thibs
#2 Collins
#3 Pop
#4 McHale
#5 Vogel
#6 Corbin