Like the title. What games are made better with mods, and foster a great modding scene that retains momentum and delivers excellent content?

My personal favorite is Fallout: New Vegas, but games like DOOM, Half-Life 2 (really all source engine games), and Minecraft have extremely unique mod scenes. Of course, Skyrim remains a modding titan. STALKER: Anomaly is already an amazing mod project and has mod lists for it like GAMMA.

What are your favorites?