I consider Mask of the Betrayer the first high quality (non-independent) RPG campaign since 2004 (Vampire: the Bloodlines). Unlike the lackluster original campaign (NWN 2), and its predecessors (NWN and its expansions), Mask of the Betrayer features an interesting story, compelling characters, well written dialogue, interesting setting, and last but definitely not least, plenty of choices and consequences.
NWN SOTU was interesting, but forgettable. The fact that the campaign wasn't "epic" was refreshing, but I can't remember much more about this game, other than that I played it once, found it playable, but mediocre, and forgot about it.
NWN 2 OC was a disappointment (as expectations were high) - some characters seem like bad copies of the Planescape: Torment characters (every attempt to create similar characters to PS:T characters is destined to fail, as they will always pale in comparison to the originals) and the story is mediocre, but there are some characters, moments and dialogues that considerably improve my impression. This wasn't a bad game, but wasn't very good either.
NWN HOTU was, apart from epic character advancement, quite boring, as it was full of generic NPCs, dialogues were low quality and campaign was incessant hacking and slashing.
Saving the worst for last, NWN the original campaign has to be the worst game I actually managed to finish. No choices, no consequences, dialogues and story that were probably written by someone failing English, awful henchmen system, just to state some flaws. I understand that this game was designed for multiplayer and single player campaign was an afterthought, but they should have dropped it entirely.
So, if I had to sort these games according to their quality, the list would look like this:
1. NWN 2: Mask of the Betrayer - excellent
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(big gap)
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2. NWN 2 - mediocre
3. NWN: SOTU
4. NWN: HOTU
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(big gap)
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5. NWN - bad
