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?
The older Total War games have a ton of great mods, Medieval 2 in particular. There's a ton of amazing mods, both historical (like The Italian Wars for pike-and-shot action) and fantasy (like the LOTR mod, Third Age: Total War).
The various Paradox grand strategy games are great too.