Well, I wanted to wait a little bit before expressing my own opinions:
I absolutely adore the original module, I6. It was the first time I ran across an adventure where the bad guy didn't sit idly around waiting for the PCs to come to him. And, just a couple years prior I'd played Dracula in our school play, opposite my best friend who had played Van Helsing. In my D&D campaign, we played Ravenloft & Griffon Hill simultaneously. My players STILL hate & fear the original Strahd and the Realm of Mists to this day.
I liked the Core map of the black boxed set, but it was clear that the designers were just scratching the surface and getting their legs what Ravenloft should be with the initial set. Most of the dark lords had interesting stories, but a good number lacked the punch to be fear-inspiring without liberal use of fiat to keep them around. They also tried to toe too tightly to a gritty style of play that even a 5th level spellcaster could easily disrupt, and a realm in which the demihuman races of the time felt badly out of place (and generally unwanted).
The Revised boxed set, despite the addition of the Shadow Rift was a good reorganization of the campaign set, though it still suffered from xenophobia towards demihumans and spellcasters. There were attempts to shoehorn in firearms, but because they weren't universal to the lands, it was a poor fit - they were better regulated to Masque of the Red Death.
Domains of Dread was a further refinement that was more player-facing than the previous two attempts, but did an overall good tightening up of the dark realms as a campaign world that could stand as full-blown campaign realm that the characters adventure in from start to finish. Still, the attitude towards magic and ignoring higher level abilities made it difficult to run higher level adventures and still make the world naturally fear-inspiring.
White Wolf/Arhaus/Sword & Sorcery did an excellent job removing the unintentional camp from the domains and making it a really dark and foreboding place. It was dense though and sometimes went into too much detail that made game play stumble. It did do a better job incorporating magic, and being aware in a way that running it was easy to keep characters on their back foot.
I initially enjoyed Expedition to Castle Ravenloft upon reading it, but got frustrated when I started to run it. While it did expand the old module into a full-fledged campaign, it suffered from the 3.5 late edition syndrome of "behind every door is an encounter!". Where the original module was sparse and created tension with its abandoned feeling, this thing overwhelmed with unending encounters. And swung away from gothic into lovecraftian in an unsatisfying manner.
When Curse of Strahd rolled around, I immediately picked it up and ran it. Though I just used the castle portion, I had a blast with it. The realm of Barovia finally felt like it fit in typical D&D world with wizards and clerics, not just fighters and rogues. The party could - and should - use their magic to their full extent and it still be a challenge without them getting lynched on piled on for using sorcery. And it was fun ... and a little bit terrifying. My players STILL fear the Dread Realms, and that makes me feel like I've done the job right.
