I think that the Might & Magic Series was the hardest. I love BG etc, but I took 4 months to beat M&M. And I played damn near every day to do it, too. It was my first first person kind of game, though, so it was slow going in the beginnings . . . but worth it . . .
BK
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The Black Storm Cloud of the Night
PartTime Bartender of Moon Hope's Inn
Dry Humor Deliverance of the Laughing Hyenas
Captain of the Knights of the Golden Dragons
3 Man for Life