I'm very much partial to Dyson Logos' Dyson's Delve, the "mini-megadungeon". 11 levels, including a couple of sub-levels which are on the same map as a full level, but can only be reached by a separate connection from a higher or lower level. But the levels are small- each fits within a 20 x 20 grid of 10' squares. Super simple statting originally for Labyrinth Lord, but that's obviously perfectly compatible with B/X, and can be re-statted to your edition of choice with minimal effort.
Inspired by Paul Siegel's running of it in two epic long weekends in 2018 & 2019, and the Wandering DMs' broadcast (the videos are on Youtube; about 24 hours of play) of the second half of that play online, plus Paul's simple faction analysis and the rumors he created for it which are available on his website, I've been able to run a really straightforward beer and pretzels dungeon crawl since about the end of March. I've had a couple of small dungeons for side-treks and stuff, but (especially stealing liberally from Paul) most of the work was done for me, and I've kept two small groups of players entertained with it as the campaign tent pole for several months now. As of Monday night the players are right in the middle of the climactic final level, and it's been a ton of fun.
I had kind of wanted to do this for a while, but being able to download the maps from Dyson's site and upload them to Roll20 was how I taught myself to run on Roll20 and make it manageable, when directly adapting my existing 5E game seemed like too intimidating a prospect.