Here follows a list of many of the well-known module series. There is a poll on the right of this site wherein you can vote. The poll will remain up until the end of the month. If there is a series that you feel is better than those I have listed feel free to make mention of it in the comments.
|Scourge of the Slavelords A1-A4|
|Queen of the Spiders G1-Q1|
|Desert of Desolation I3-I5|
|Rappan Athuk 1-3|