Brian: One of my favorite is Uda Tyygk, in the Iron Wind. Hidden fortress, the Thyfuriak. Very cool. Reminds me of a toy I used to have: the mountain fortress from MAC (mobile action command)
Peter: For me it has to be the MERP campaign book Northern Mirkwood. This book has everything, the floor plans vary from the great halls of Erebor to towers and orc holds, every one of them I have reused time and again. The master military charts with every NPC, and class or adverary clearly detailed make off the cuff encounters dead easy and the amount of unique content to make the region really stand out as being different from any other woods or forest. This was also the first MERP campaign book I bought and with my only other experience of ‘modules’ being D&D ones, this book completely changed my concept of what a rpg suppliment could and should be.