Answered February 28th, 2019

Drew Brees finished 2018 with a 74.4 completion percentage, breaking his own record for best in a season in NFL history.

Which QB has the highest completion percentage in a season?
Interpreted as:
Which QB has the highest completion percentage with a minimum of 14 passing attempts per team game in a season?
2018Brees2017Brees2016Bradford2011Brees2009Brees1982Anderson1945Baugh1994Young1989Montana2018Cousins2016Brees2014Romo2016Ryan2015Cousins2018Wentz2013Rivers2018Ryan2004Griese2014Brees2004Culpepper74.472.071.671.270.670.670.370.370.270.170.069.969.969.869.669.569.469.369.269.2
NAMESEASONTMGPPCTCMPATTYDSAVGYDS/GTDTD%INTINT%SCKSCKYRATE
Drew BreesDrew Brees2018NONO1574.43644893,9928.2266.1326.551.017121115.7
Drew BreesDrew Brees2017NONO1672.03865364,3348.1270.9234.381.520145103.9
Sam BradfordSam Bradford2016MINMIN1571.63955523,8777.0258.5203.650.93727699.3
Drew BreesDrew Brees2011NONO1671.24686575,4768.3342.3467.0142.124158110.6
Drew BreesDrew Brees2009NONO1570.63635144,3888.5292.5346.6112.120135109.6
Ken AndersonKen Anderson1982CINCIN970.62183092,4958.1277.2123.992.92615495.3
Sammy BaughSammy Baugh1945WASWAS870.31281821,6699.2208.6116.042.2109.9
Steve YoungSteve Young1994SFSF1670.33244613,9698.6248.1357.6102.231163112.8
Joe MontanaJoe Montana1989SFSF1370.22713863,5219.1270.8266.782.133198112.4
Kirk CousinsKirk Cousins2018MINMIN1670.14256064,2987.1268.6305.0101.74026299.7
Drew BreesDrew Brees2016NONO1670.04716735,2087.7325.5375.5152.227184101.7
Tony RomoTony Romo2014DALDAL1569.93044353,7058.5247.0347.892.129215113.2
Matt RyanMatt Ryan2016ATLATL1669.93735344,9449.3309.0387.171.337235117.1
Kirk CousinsKirk Cousins2015WASWAS1669.83795434,1667.7260.4295.3112.026186101.6
Carson WentzCarson Wentz2018PHIPHI1169.62794013,0747.7279.5215.271.731202102.2
Philip RiversPhilip Rivers2013SDSD1669.53785444,4788.2279.9325.9112.030150105.5
Matt RyanMatt Ryan2018ATLATL1669.44226084,9248.1307.8355.871.242296108.1
Brian GrieseBrian Griese2004TBTB1169.32333362,6327.8239.3206.0123.62616997.5
Drew BreesDrew Brees2014NONO1669.24566594,9527.5309.5335.0172.62918697.0
Daunte CulpepperDaunte Culpepper2004MINMIN1669.23795484,7178.6294.8397.1112.046238110.9
Troy AikmanTroy Aikman1993DALDAL1469.12713923,1007.9221.4153.861.52615399.0
Chad PenningtonChad Pennington2002NYJNYJ1568.92753993,1207.8208.0225.561.522135104.2
Derek CarrDerek Carr2018OAKOAK1668.93815534,0497.3253.1193.4101.85129993.9
Marcus MariotaMarcus Mariota2018TENTEN1468.92283312,5287.6180.6113.382.44224392.3
Tom BradyTom Brady2007NENE1668.93985784,8068.3300.4508.781.421128117.2
StatMuse has season-level data for completion percentage and pass attempts per team game going back to the 1932 season.