I must name as the true successor to the original X-Com....
UFO Alien Invasion
Base building, Research, Fully 3D Randomized Maps, and,
of course, Old-School-Turn-Based-Tactical-Goodness.
The UI has a few rough spots, still, but I have already
discovered some great additions to the original.
PS. I'm dying to try out the multi-player on it...anybody?