The Lost City - 24 - 2 = 22 get lost you poser
Desert of Desolation - 31 +1 = 32 you are one hot ….
Isle of Dread - 21
Castle Amber - 24
Red Hand of Doom - 15
I specifically didn't include Saltmarsh (or some other great modules) because they've been adapted for 5e. This is really an exercise to see which older module is the most popular, as WotC have started a pattern of updating old ones into bigger books (Curse of Strahd, Tales from the Yawning Portal, and Ghosts of Saltmarsh).
The Lost City - 23
Desert of Desolation = 32 Isle of Dread - 22 - 2 = 20
Castle Amber - 22 Red Hand of Doom - 13 + 1 = 14