The toughest runs to win a European Cup/Champions League are the following in my opinion:
Inter Milan 63/64 EC: Everton, Monaco, Partizan Belgrade, Borussia Dortmund, Real Madrid
AC Milan 68/69 EC: Malmo, Bye, Celtic, Manchester United, Ajax
Ajax 72/73 EC: CSKA Sofia, Bayern Munich, Real Madrid, Juventus
Manchester United 98/99 CL: LKS Lodz, (Group: Barcelona, Bayern Munich, Brondby), Inter Milan, Juventus, Bayern Munich
Real Madrid 99/00 CL: (Group: Molde, Olympiacos, Porto) (Group: Bayern Munich, Dynamo Kyiv, Rosenberg), Manchester United, Bayern Munich, Valencia
Real Madrid 17/18 CL: (Group: APOEL, Borussia Dortmund, Tottenham), PSG, Juventus, Bayern Munich, Liverpool
Real Madrid 21/22 CL: (Group: Inter Milan, Shakhtar, Sheriff), PSG, Chelsea, Manchester City, Liverpool)