Difficult to rank them, as there are so many I like. Here are some of my favs:
B2: Keep on the Borderlands.
B4: The Lost City
B10: Night's Dark Terror
X1: Isle of Dread
However, as my top favorites I'd probably have to choose X4-5-the Desert Nomads series. Those two modules had several varied encounters, introduced several interestng new monsters, had some challenging puzzles, roleplaying opportunites, and an epic sweeping plot.
A couple of modules that might be worthy of some praise:
X7: War Rafts of Kron -- always liked it. One of the few Underwater adventures.
X9: Savage Coast. This module spurred a major setting, though the module itself is probably not a particular classic.
XL1: Worth it for the stats of Warduke and Strongheart alone.
CM1: Test of the Warlords
CM9: Legacy of Blood. Module set in an interesting rustic locale with a nice plot.
DA1-4: Blackmoor series