7-0: Bayern München vs Basel, 13/03/2012, 1/16, second leg
7-0: Bayern München vs Shakhtar Donetsk, 11/03/2015, 1/16, second leg
Source: UEFA.com
If you include European Cups as well, then the win with the biggest margin is:
11-0: Dinamo Bucharest vs Crusaders, 03/10/1973 first round, second leg
Source: theguardian.com