Honestly? LA 2.1 has stuck with me throughout the years as a mod I always return to. It's nostalgic and reminds me of times when things weren't so complicated. It's also one of the biggest staples in the American Modding Community as well.
German-wise, it would be Fuchsburg and BFE:MP. They are two of the biggest revolutionary non-campaign-based mods that changed how some people viewed modding.