I was always partial to the UK series of modules, especially The Sentinel, and it's followup, The Gauntlet. Great times.
Most of the UK stuff was cool if a little whacky. If I remember correctly the Sentinel and the Gauntlet took place on teh border of the Hold of the Sea Princes. It had to do with a mountain pass the was protected by the wearer of the sentinel, which made defense pretty easy, and a Ogrillon(?) who came in possesion of the Gauntlet which was made to defeat the Sentinel. A very fun set of adventures.
The other UK module that I rember fondly was "When a Star Falls". With a little work it was great. I found teh UK stuff to be very good on teh whole, with great story plots, but they needed a little "Beefing" up.
Other Favs of mine were:
I series: All the Desert of Desolation, and really fond of teh Dwellers of the Forbidden City. Great time that one.
A Series: What a fun campaign that was. Who doesn't like kick'n butt on slavelords.
U Series: Underwater adventure at its finest. Sahuagin are just nasty!
S Series: The ones already mentioned, plus the Lost Caverns of Tsojcanth. Now that was just fun to play.
All I can think of off the top of my head.
