Gotta say Fallout but I'm a big fan of most of your games. Loved the Star Trek 25th anniversary game, the LOTR rpgs, Earthworm Jim. I recently picked up MDK and Fallout tactics from gog.com so well see how I like MDK soon.
Aye, Fallout is a game which has a lot of fond memories for a lot of people and is still actively played by a lot even today. Personally I loved the overall feeling of the game and it's sense of humour. Die roller of Doom, Beware my fumbles.
Such a limited selection - But still full of great games, by far though it is ofcourse Fallout that I like the best. Isometric, turn based, roleplaying - It has it all! I am so glad I'm here, I mean really!