Scandinavia has been my first choice for many years. There is a good ratio of small curvy roads and highways, beautiful sceneries and it looked different from old map parts like Germany, Going East,... at the time of release. Especially Norway fascinated me the most, beautiful fjords with big mountains and the tunnels look amazing there. In my opinion it was the first modern DLC and very timeless at the time of release. This DLC fascinated me so much that I would like to visit this region (especially Bergen) in real life.
Then I combined it with Beyond the Baltic. It expanded the available area in this region and brought a lot of small roads with the feeling of the countryside.
For now, when I want to relax from D-C and drive in some DLC, I go in Scandinavia + Beyond the Baltic Sea or Road to the Black Sea + Western Balkans. I would choose Scandinavia if I want its own unique atmosphere or Road to the Black Sea + Western Balkans if I want the freshest and most detailed DLC. Keep in mind that this DLC contains a lot of small roads and brings a lot of rural atmosphere, so if this is not your cup of tea, look around for other DLCs.