In White Dwarf #63 there is a modern day adventure called "Draw the Blinds on Yesterday" (that is modern = 1980s). I'm not sold on the scenario as a whole, but it has a truly kick-ass scenario starter.
Flight 1742 left Athens and disappeared [en route to London]. At first a communications failure was suspected, then it was assumed that the aircraft had crashed without warning. The plane was given up as lost. Now it has reappeared after three weeks, with the Captain apparently unaware of the passage of time...
The investigators are all on board, and the trick is determining what happened to those three weeks, and where everyone went. As a one-off, or a campaign starter, this is just peerless, a way to instill not only instant suspense and mystery (have I changed? who else was on board? what does Greece have to do with this?), but also role-playing opportunity (what would it be like to have vanished from job and loved ones?), but also perhaps the most key thing of all -- a brilliant way of roping disparate characters together.
Too many CoC scenarios fall into the rut of "someone old/rich/you know contacts you to do some quest/search a house/open a box/deliver something." What are some of the greatest, most useful, most ingenious scenario or campaign starts you've ever seen?