Hmm ...
#1 Euskadi (Basque Region of Spain + France)
#2 Catalonia
#3 Kurdistan
Then some toss-ups for #4-5:
- Hannover (Lower Saxony + Hamburg)
- Aquitaine (🖕France)
- Manchuria
- Hong Kong
- Macao
- Puerto Rico
- unfuck some parts of Africa (so many to choose from)
I'm leaning towards...