Honestly, there's quite a few recent classics for me.
The Adventure Anthologies have mostly been classics for me, especially Journeys Through the Radiant Citadel, which was as much a setting/adventure sandbox as it was an adventure anthology. Infinite Staircase is similiar for me, introducing a new (or newish) element to the cosmology of D&D, but because it relies on more classic adventure modules as its anthology pieces, it doesn't quite hit the classic mark for me.
Vecna: Eve of Ruin I haven't run yet, but it feels like a classic to me, because it sort of serves as a climactic "final boss" adventure path to 5.1 D&D, taking us on a tour of the major settings and adventure season locations previously visited in 5e and also confronts us with some of the most iconic D&D heroes, villains, and power players of the in-between, and it even delves (spoilers ahead) into the mythology of the Rod of Seven Parts and Mishka the Wolf-Spider & the Queen of Chaos, something only alluded to in 4e's setting books like Demonomicon but also the subject of fantastic lore in late 3.5e's exploration of the Obyriths, and something dating back to the very beginnings of D&D but using all the lore that has built upon it since.
I REALLY want to run this adventure, but it's sort of 5e's take on an Epic Tier adventure path (to use 4e parlance), and while I want more such modules to allow us to fill out the highest tiers of game play, it's also just really hard to get a group to go for high level play given the complexity therein.
While I LOVE the Spelljammer and Planescape three-book sets, their adventure modules are by far the weakest parts of each. I do like that the Planescape one dealt with time travel, it just is very difficult to play around with.
Essentials Kit was an instant classic sandbox and I've run it multiple times, even for one-character play using the sidekicks rules included (though I've used TCOE as a support text since as that book updated those sidekick rules ever so slightly). I don't love FR, but Essentials Kit made the FR feel legit to me and not just a world dominated by powerbroker NPCs with the PCs playing in the margins. Probably feels right because it's such a low level adventure and a sandbox at that.
The Wild Beyond the Witchlight was definitely an instant classic to me; it's what I wanted of a Feywild adventure in 4e when we got "just" Heroes of the Feywild and not a companion adventure/setting a la Shadowfell: Gloomwrought and Beyond for Heroes of Shadow or as The Plane Below served for Heroes of the Elemental Chaos or Underdark and Menzoberranzan served for The Dungeon Survival Handbook. Moreover, Witchlight is just a really good adventure path and can be used with or without the larger Feywild adventure path.
Finally, I want to give a shout-out to Strixhaven: Curriculum of Chaos. It's just as much a setting book as it is an adventure module, but unlike its older siblings Guildmaster's Guide to Ravnica and Mythic Odysseys of Theros, it leands more heavily on the adventure module side of the coin than on the setting details side. In that regards, it fits more in line with say, the seasonal adventure modules like those for Ravenloft, Chult, Waterdeep/Undermountain, Baldur's Gate/Avernus, or Icewind Dale. No, it's not a setting guide book, but adventure first and foremost, but the setting brings something unique to the table that isn't otherwise easily playable. In this case, Strixhaven gives us the magical school fantasy, and it does so in a way that steps out of the shadows of HP. It's fresh, and I'm pretty sure was designed for both MtG and D&D at the same time.