Accuracy
Deshaun Watson is the single most accurate Quarterback who's ever lived. No, Seriously.
Quarterback Accuracy, Career
Dak's up there too, but not where Deshaun is. And Dak's accuracy is actually going down as he's asked to throw it more. 65.9% his last 2 years, vs 67.8% his rookie year.
Proficiency
Proficiency is the point where efficiency meets prolificity. We've established Deshaun is highly accurate, but in 2020 he had a historically prolific season as well. He completed 70% of his passes in 2020, which does seem to be a recurring achievement amongst QBs the last 5 years or so. But not at the crazy rate he was doing it: Deshaun Watson led the entire NFL in yards per attempt in 2020, at 8.9 It is the highest yards per attempt for a 70% completion passer in NFL history. He wasn't dinking and dunking, he wasn't playing it safe, he was straight up bombing the ball downfield... and maintaining 70%. For comparison's sake, Drew Brees never had a YPA season higher than 8.3.
Additionally, Deshaun didn't exactly have a Dallas offense around him. The only team's QB's who were sacked more than DW were Wentz/Hurts. 31st in pass protection. Texans were also 31st in rushing yards, you knew what they were going to do on offense. Also don't forget this is the season AFTER they traded away the NFL's best WR. Led league in ypa, yards, completion percentage. Drew Brees is the only other quarterback in NFL history to throw for 4800+ yards in a season at 70% completion. For comparison, Dak's 4900 season in 2019 was at a 65% clip. Craziest part of this stat is Deshaun was only 13th in attempts! He is the only QB in NFL history to lead the league in passing yards despite not being in the top 10 in attempts. In a season where Brady played like he did, Rodgers played like he did, Mahomes played like he did, it's pretty nuts.
Deep Ball Passing
We'll stick to 2019 for this one, as it was Dak's last full season, Dak's best season, and not Deshaun's impossible 2020 to make it fairer. Dak was heralded for his deep ball passing in 2019, being amongst the league's leaders, but lets look at how that played out a little deeper.
Above we see Deshaun amongst the top of the league in deep ball accuracy, but Dak is actually not too far behind. However, this is a list of 20 yards or greater, what is generally considered a deep ball. If we move the sticks to 30+ yards, it looks a bit different:
Deshaun remains amongst the most accurate QB's at throwing the deeper passes (far right column), while Dak drops all the way between Jacoby Brissett and Mitchell Trubisky. If you notice the green column in the middle, thats passes between 20-30 yards, and Dak's completion percentage there is unusually high... that's what's inflating his overall deep completion percentage. If you want a 23 yard post, Dak is as good as any. But if you want to go for the big one... also notice Deshaun has 20 completions of 20+ yards in 2019, easily the most of anyone. He's really good at it and he knows it.
Probably some other categories where Deshaun looks pretty good, but these are the 3 that stand out the most to me.