I found my way into the Mists in 1997 when, almost by chance, I bought the Realm of Terror boxed set from an RPGer that was selling his stuff and leaving the country (I also got Darklords with it). It was an eye-opening experience reading through it for a younger me that had gotten into RPGs that very same year. That's why the Black Box will always have a special place in my heart. But I consider Domains of Dread, which I got maybe a year later, to be the definite 2e setting sourcebook, as others had said in this thread. It was the first Ravenloft product that treated the mist-bound realms as a real setting, where domains interacted with each other in a more coherent way and players could be natives with more stakes than needing to escape the so called "weekend in hell". The Red Box I didn't buy until maybe ten years after DoD, and mostly to complete my collection, but I can understand that back in 1994 it must have had a very real impact on the fan base (except for the maps with their wrong scale and the distance issues it brought).
As for other 2e books, without a question the Van Richten's Guides are the real standouts. I also got my first one (the Guide to Vampires) mostly by chance, while perusing through the limited stock of Ad&d products a local comic book store had. Needless to say that it was revolutionary to me. All the other guides were as well, but the Guide to Ghosts, Guide to Fiends and Guide to the Vistani stand out in my memory as my favorite ones.
Special mentions go to the Castles Forlorn boxed set, Champions of the Mists and Carnival. As for adventures, the aforementioned Castles Forlorn reads like an awesome adventure (but also a nightmare to run), Servants of Darkness is really good (and I even got to run it!) and I quite like The Shadow Rift, even if many fans find it to their tastes. And Bleak House is simply epic.