I found this an interesting question, since when I thought about it I realized that the modules I have most frequently replayed were not necessarily the same ones I would mention if asked to list the very best NWN modules I had played, so be warned that a few of the following modules do have significant flaws of one sort or another. I nevertheless found each to be very enjoyable to replay with a variety of different characters.
Sapphire Star is probably the module I have played more times than any other.* Why? It is very non-linear, and it has lots of character-specific content like class-specific quests, different paths for different alignments, semi-hidden "easter eggs" and other such things that make a replay interesting. Most importantly for me, it also has extremely difficult combat making it an interesting venue to try out different builds and tactics. On the down side, the story elements are confusingly presented, much of the dialogue is poorly written, and the combat difficulty can assume unfair and ridiculous proportions if you do not do certain quests in the right order (an order you have no apparent way of knowing in advance).
I have also replayed the Blackguard trilogy a good number of times for a similar reason - its high difficulty level makes it very interesting to try out different builds and tactics in it - even though I found the utterly simplistic role-playing of this series rather disappointing, and for obvious reasons one's choice of character types has significant limitations.
Honor Among Thieves is another module that I have found it very enjoyable to replay. It is non-linear and offers a variety of different role-playing paths that can have dramatically different endgame consequences. As one would expect of a module by Baldecaran, it is also one of the all-around best NWN modules out there.
The Defence of Fort Tremagne is another module with lots of different options for different types of characters that I found worthwhile to replay a number of times. It is one of NWN's more underrated and unfairly overlooked modules IMHO, though admittedly it does have some potential for buggyness at certain points that seems to have understandably turned off a number of players.
Some honorable mentions (so-called because I have not gotten around to spending as much time actually re-playing these modules as was the case with the above, but found them well worth trying more than once):
Bloodright: The Blood Royal is a well-done non-linear module with lots of content specific to certain character types.
The Saleron's Gambit series offers some different paths for different character types, and is an all-around good series.
Magical Master's Siege of the Heavens (which appears to not currently be available on the new vault) offers some very interesting and unusual (at least for NWN) combats that are worth trying with different character types.
*Technically, I have actually played my own modules more often than any other, by a huge margin, but like Tarot I assume that you do not intend to count play-testing.
