What made Strahd do great for you? What were your personal highlights?
This is a good question. I apologize in advance for the book-length response you probably didn't ask for.
The overall vibe was a major part, of course. Just a great, classic story, dripping with gothic horror. It's the only game where we put together a playlist, and it was so easy to do. I borrowed from the Vampire the Masquerade TTRPG and video games as well. And Strahd is a great villan. He's not stupid, he has believable motivations, and he's wrong. And he'll never accept that.
But the D&D elements also came together well: most of the writing was cohesive and contributed to the theme (I cut some incongruent bits, like the lich temple). The "dungeon" was this impossible labyrinth of a castle but thanks to the excellent maps I could still follow it. And it could be attacked from any angle! I don't think there's a more open-ended way to approach a dungeon. And however you came at it: it was a challenge.
It also had a massive sandbox outside the castle that was perfect for setting up the villain and expanding the lore. I love the overall theme of trying to do quests and level up all while in the shadow of the primary threat, visibly foreboding at all times, which you know you cannot possibly handle without something - anything - that can help.
I think it's a good example of freedom within constraints. You have a small area that you can do a lot with, but you can't go far - and you know you're never truly safe. This forces you to deal with what you've got.
I'd say the biggest difference between CoS and something like Vecna or Tiamat is that CoS set up a world with points of interest, but then you had complete freedom to do whatever you wanted inside those walls, and then see how that affected everything else. Your could be creative. There was no clear path. Your actions had reactions. Tiamat and Vecna are just effing railroads. You're being shuffled around from point to point like a theme park ride. And you can't leave the plot because guess what: it's all a veneer. There's no world reacting to you. There's only 1 path, and your "choice" is to follow it or stop.
So I guess the tldr is: well-supported player agency within a cool setting. A solid sandbox and dungeon that encourage creatively exploiting that agency. And a realistic, memorable villan who made it all personal.