Giants: Citizen Kabuto - A fun goofy game with vivid visuals and mechanics that were ahead of it's time.
Clive Barker's Undying - One of the best, if not the best, horror fps game made. Had a really cool and unique setting and mechanics. It was also the best use of Unreal Engine 1.
No Ones Lives Forever 1 and 2 - One of the most unique FPS series ever made.
The Longest Journey - I'm well aware of Dreamfall and Dreamfall Chapters, which I personally thought were disappointing compared to TLJ, those don't count. I mean a
real sequel to TLJ in the form of a charming point-and-click adventure. One was supposedly going to get made if Chapters hit a stretch goal, but that never happened.
Unreal - No one ever mentions the
original Unreal. I would love to see a proper sequel that the game deserved, since the Unreal II we got almost 15 years ago was such a disappointment.
Freedom Fighters
Finally, I'll echo others sentiment that we definitely should have gotten a real sequel to Prince of Persia 2008, not that handheld one that was made for the DS.