I picked the original module, as it was just so damn cool. I have a lot of love for the Realm of Terror boxed set, though (which was for AD&D 2e, not AD&D 1e, by the way ). The other stuff was also good - Ravenloft had very few duds in its product line - but the original has a special place in my heart and gaming library. It's the adventure I always turn to when I want to introduce people to D&D as a hobby.
I6 Ravenloft. "Expedition to..." wasn't bad either, though it was marred by the wretched 'delve' format (though that format was implemented best in Expedition).
I chose Domains of Dread, myself. For me, this was as complete as Ravenloft (the setting) ever got under one cover. I often wished that TSR would have appended some basic play rules, just so the product would have achieved true standalone status.