AC Milan 1988-1994 were an absolutely amazing side. Maldini, Baresi, Costacurta, Gullit, Rijkaard, Van Basten, Donadoni, Boban, Savicevic, Desailly, Panucci, Massaro, Ancelotti, B. Laudrup and Papin all playing for them throughout that time. They were just a machine of a side and when you consider what a lot of those players went on to achieve after that period too.....well it is mindblowing.
The team that won the 1993/94 Champions League was just incredible and I remember them absolutely destroying Johan Cruyff's Barcelona in the final, 4-0 and that despite all the experts saying that Milan were ageing and couldn't compete with a vibrant Barcelona side that had Romario and Stoichkov banging goals in left, right and centre and a side that had smashed Man Utd to pieces in the group stage at the Nou Camp.
Serie A Winners: 1987/88, 1991/92, 1992/93, 1993/94
Runners Up: 1989/90, 1990/91
European Cup/Champions League Winners: 1988/89, 1989/90, 1993/94
Runners Up: 1992/93
UEFA Super Cup Winners: 1989, 1990, 1994
FIFA Intercontinental Cup/World Club Cup Winners: 1989,1990
I think the Barca side of recent years would run them close in fairness but AC Milan dominated home and abroad and I think are the last team to successfully retain the European Cup and got to 4 finals out of 6.