A significant measure of a quarterback is how he performs on third-and-long when everyone knows a pass is coming. By a number of statistical measures, Clemson's DeShaun Watson is the best when throwing in the ACC on third-and-7 or longer. The invaluable website cfbstats.com breaks down quarterbacks in a number of situations, including performance on third-and-7 through 9 and third-and-10 or longer.
The following is a compilation of how ACC quarterbacks have done on third-and-7 or longer. The sample size is not considerable, but, rather remarkably, of the 14 quarterbacks, 10 have attempted between 34 and 41 passes, so at least there's a degree of consistency in the data.
On third-and-long, Watson has the best yards-per-attempt average (11.8), highest rating on third-and-long (194.0) and the highest percentage of conversions (42.9). (I ranked the quarterbacks by that last statistic, as I thought it was the most telling - who kept the chains moving most frequently on third-and-long.)
Georgia Tech quarterback Justin Thomas has the most touchdown passes on third-and-long (six). Thomas' passer rating on third-and-long is second behind Watson (179.6) and his 19.4-point differential between his overall rating and third-and-long rating is third highest. (If you're wondering, the differential isn't a true differential. I subtracted the ratings that I rounded off, not the actual ratings. I hope you'll be able to forgive this insanity.) A.J. Long of Syracuse leads that category (31.0, although it helps to have a lower overall rating).
Virginia Tech's Michael Brewer has the highest completion percentage (59.1) and second-highest differential. It's noteworthy particularly because he has by far the most attempts on third-and-long (64), because his completion rate (59.4) is just a hair under his overall percentage (61.9) and also because the Hokies offense is a bit of a mess.
Florida State quarterback Jameis Winston is not first in any category, but had the third-highest conversion rate.