Misery Stone. I don't think it's toppable. I played straight through and couldn't step away except for the bathroom. I even ate at my computer. But there are a very, very few bugs.
A walkmesh issue by the shambling mound, you can walk into a corner of the wall. A walkmesh issue with the two giant statues in Castle Firglore library (can walk right through them).
Typos and description problems. The artifacts of Firglore have as their description "There is nothing special about this object.". As artifacts, there should be some sort of description. A minotaur whose description is a gargoyle. The quarterstaff shovel. Hand drawn picture keys. "Machince", and "stready". The incomplete description of "This chair".
Finally there was a broken journal entry. Though this may be related to my overrides, I got the same broken empty journal with Trinity. I'm using Tony K's ai and Kaedrin's prc pack.
edit: More bugs:
One female companion is a Vistani gypsy and native the Firglore. The other female companion talks about the Vistani. We never find any other evidence the Vistani exist even though we travel the length and breadth of Firglore.
A red dragon, what was that doing there? Make it a white please. There's one on the Vault.
Dynamic journal failure. The journal keeps track of your good/evil deeds. This only worked for the very first deed, in the inn with the sleeping guard. After that I was stuck on 1 good/0 evil the rest of the game even when I did some definitely good deeds (the Misfield children).
Journal failure: Giving a female companion the items for her quest would update her journal. The update made no changes to it. I only did this after I had completed her quest.
Duplicate npcs: In the basement of the asylum, there were duplicate liches. The second one did not go hostile, but did attack until I'd killed the first. Once I killed the first I could talk to the second again and get the same conversation as the first. I couldn't attack it again though, it would not go hostile.
Finally, a preference. There are several item accumulation quests. It would be nice if the items were removed from the player inventory after they are no longer needed. I was walking around with a number of keys.