Has to be Masks of Nyarlathotep for me. As has been mentioned the first chapter of Shadows of Yog-Sothoth - the Hermetic Order of the SIlver Twilight and The Pale God from the Great Old Ones were stand-outs as well. There are just so many, so much quality!
I read somewhere - YSDC I think - of a bastardisation of some of the main campaigns. Basically whoever it was, was running Masks with addons. In came Silver Twilight which turned out to be a(nother) front for NWI, Jeremy in the well, Castle Dark, Mountains of the Moon and several other mainstays. Hell of a lot of work for the keeper, but what a great game to play in.