Ravenloft Red Box. Dripping with evocative flavour, giving you a fairly comprehensive look at the setting with some of the initial bad domain design smoothed away plus the tarokka deck. Never equalled.
The revised DS box is really cool for the expanded area it covers, but the original has a feel and atmosphere that is unmatched for me, a lightly sketched world just dripping with adventure. We're over thirty sessions into streaming a run through the classic DS adventures on Twitch at the moment and it has been great to see the world come alive for players who are familiar with the setting and those who were brand new to Athas when we started. Such a great piece of design work
For me it's Introduction to Advanced Dungeons & Dragons (both versions of it)/First Quest. That thing was published under like three or four different titles and versions. But it was cool. It had minis, a map some cool cards and best of all a very campy, ultra-cringe audio CD.