Even though they never made it into a supermodule. I think the Saltmarsh series (U1-3) would be the best.
Great game balance, Great Surprises.
They really gave players a chance to use their brains.
Follow U1-3 with I7 Baltrons Beacon for more fun in the same vein..
T-4 followed by S4 (Lost Caverns of Tsojcanth) and then the GDQ series
and finally
S1 is another great campaign. More combat than thinking but still good.
I always skipped Q1 in the GDQ series as I though it to be a very very poor module..it made no sense..giant spider ship..yeah right..and for a major deity of the drow LLolth was a pussy cat. It was so much better to end the series with
D3 by capturing LLolth (after a BIG battle) in the Prison of Zagyg gained in S4 (for a short time anyway).