Notice I didn't say favorite. Best and favorite often don't align, especially with questions like the one being posed.
See, my favorite module was G1-3, the giants series, but it was far from the best or most useful. My most used module was T1-4, Temple of Elemental Evil.
So, what's the best "classic" module in my opinion?
B2, The Keep on the Borderlands. Sessions of gameplay within. Setting and dungeon all rolled into one. Damn shame I didn't get to appreciate the greatness of B2 until my later years, when everyone and their brother had already spent hours or days exploring it.
So, what's the best "classic" old school module in your opinion?