Most people would consider Neverwinter Nights old news along with Shadows of Undrentide and would rather focus on Hordes of the Underdark but I prefer to take a more holistic approach, to judge the game in total for each expansion not only introduced new adventures for the players but also new functionality in the tool set, new character options for developing a fairly unique character. If you plan on getting Neverwinter Nights you might as well get the whole series for you will gain a wealth of gaming opportunities.

Neverinter Nights Pros: The DM Client, Toolset, decent implementation of Dungeon and Dragons 3e rules. Wealth of character options and building capabilities.

Shadows of Undrentide Pros: Prestige classes, new skills and feats to allow further customability of characters, the new tile sets and a decent adventure to play.

Hordes of the Under Dark Pros: A fantastically written adventure even with the use of the cliche, epic levels, and new building tools for making new
adventures.​