I will say Andy Dalton. He was seen as a back-up and when pressed into service he struggled for a bit in his first 2-1/2 games but after that Dalton has really started to develop a very good report with the offense even with the patchwork offensive line:
Dalton first 2-1/2 Games (NYG, ARI, WASH; including 1 pass against Atlanta) Dallas Record 1-2 (not including game vs. Atlanta due to 1 pass attempt)
52 for 85 61.2% Comp % 452 Yards 1 TD 3 INT's Sacked 7 times QBR 70.2
Dalton next 6 Games Dallas Record 4-2
135 for 201 67.2% Comp % 1,474 Yards 13 TD's 4 INT's Sacked 11 times QBR 104.78
Dalton was also out due to injury and Covid in Weeks 8 & 9 (both losses) but upon returning Dalton has really helped turn things around for Dallas and has really been spreading the ball around to get all 3 starting WR's involved.