There are only four I've run more than once, that I can immediately think of, and only the first two in the actual edition/game they were made for (i.e. 2E). It's hard to say which I've run/played "most" because of the different levels of commitment. If it's like "technically brought out and used", it'll be Lords of Darkness, because I've often run a single adventure from it as a sort of "test-run" for a system, but in terms of time spent, it's a toss-up between Dragon Mountain and Night Below, probably Dragon Mountain narrowly wins because I've played it once and run it twice, whereas Night Below I've run part of it (it's huge) twice.
1)
Dragon Mountain (1993) - A somewhat controversial/marmite 2E boxed set with DiTerlizzi art and some really cool ideas, and really the genesis of the modern vision of kobolds.
2)
Night Below (1995) - Never run all of this, but I've run part of it, it's a huge Underdark-set campaign-thing if you're not familiar with it, very cool.
The following two I think I've only run once in 2E, but have run in other systems more than once - later editions of D&D and Dungeon World particularly:
3)
Lords of Darkness (1988) - A collection of short adventures involving undead (possibly one is wererats), set in the Forgotten Realms, quite varied and pretty atmospheric despite them being very short.
4)
Temple, Tower, and Tomb (1994) - Three kind of bad and Tomb of Horrors-but-less-so-ish adventures with a really cheesy setup that at the same time, are incredibly easy to drop into a campaign, because the setup is so cheesy and it's all so self-contained. This is where my players scientifically determined that wood does not conduct Evil, but metal does, so Evil is presumably electrical in nature.
And that, of course, would be the corollary thread to this one: what classic modules you thus far have somehow managed to miss.
Dangerous talk! Not only would the lists potentially be long but we'd have to determine what counts as "classic", and I dunno if things have changed in the last 15-20 years but I would steadfastly call, say Dragon Mountain "classic" and others would call it newfangled rubbish!