As a DM, the one I’ve enjoyed the most was running Return to Undermountain and Expedition to the Ruins of Greyhawk back to back as one story arc. It was a blast and I had a lot of fun as the DM in that arc. My second favorite DM track was running the Speaker in Dreams module. I loved the illithid behind the plot, Gharaleth Axom.
As a player, my favorites so far have been Return to White Plume Mountain, Shackled City and facing off against Lord Soth in Ravenloft (I think the module was When Black Roses Bloom, but it was a long, long time ago). The last was a blast as my wizard Delban used a whisper/message spell to speak torturous words into the ears of the slumbering Lord Soth in trying to wake him from his trance and save his dying realm of Sithicus. It was also in Lord Soth’s castle where Delban ran afoul of a banshee, which had the effect of turning him all Raistlin-ish (white hair and gold, glittering eyes – no hourglass pupils thankfully) and probably started him down his path to evil. Blame it all on Ravenloft. It tends to have that effect on people.