For me, definitely Avernum 3. Took weeks to get through all of it (even with editor-enhanced characters). Of course, it was back when I didn't have an Internet connection, so a lot of stuff I did out of curiosity and without a walkthrough. I'm currently in the middle of completing the last Avernum game (6) from the original saga, and the last 2 Geneforge games (4 and 5), but like to go back to Avernum 3 from time to time to explore it all again.
I am really looking forward to how Jeff manages the new Avernum 3 remake, especially the surface world of Valorim. With a semi-new engine, I hope there'll be more twists and things to explore on the surface.