My favorite is still 'Castle Ravenloft'. It's the most difficult and it had some great variety in the missions and boss monsters.
Both 'Wrath of Ashardalon' and 'Legend of Drizz't' just streamlined the game's formula at the cost of variety. WoA had better magic items and Drizz't had better characters but in both cases the net result was too easy scenarios.
The two latest entries 'Temple of Elemental Evil' and 'Tomb of Annihilation' fully support campaign play. While I really enjoyed that, the scenarios were still way too easy and the quality of the minis has declined.
I find they're all pretty interchangeable in terms of game play quality - and I'm sorry to admit that I've cannibalized all my games for minis to use in D&D (I own all of them except ToA). I greatly prefer Shadows of Brimstone or Descent if you're looking for a dungeon crawling boardgame.
Castle Ravenloft. This game is dynamic and diverse, even if you have already passed a scenario, you have the opportunity to go through it again, and the game will never be the same as the previous one - it is unlikely that you will meet the same monsters, or you can build a similar dungeon map.