The Milan 89-90 side were absolutely fantastic, they'd be first on my list. Not listed and probably rightly so because nothing's cast iron yet but I'm pretty sure this current Barcelona side will go down as one of the all time greats too- Messi 36 Goals, Eto'o 32, Henry 26 and then you have two of the finest midfielders in the game in Xavi and Iniesta.
Little bit of bias perhaps but I'd also like to mention the City side of the late 1960s and early 70s, the era of Bell, Lee and Summerbee under Big Joe and Malcolm Allison. Won the league, the cup and the cup winners cup in quick succession. Pity it's been all downhill from then!!