C3 was a lot of fun, and is great for when you don't want to run an entire campaign. There are some very challenging parts of that adventure.
I had a great time running C4 and C5. There was just enough Celtic flavor to get my imagination running and was the seed for a large part of my home-brew campaign world. C5 takes the adventure in an unexpected direction, but both modules are great.
I have very vague recollections of running A4 as a one-shot. I always wanted to run the GDQ series, but, sadly, have not yet had the opportunity. As a young DM, nothing fired my imagination more than the possibilities of the portals to other worlds on the top layer of the Demonweb. Caer Darrow FTW!