The answer is:

>! Switzerland

Because:

>! [Campione d'Italia](https://en.wikipedia.org/wiki/Campione_d%27Italia) is an Italian exclave which is an enclave in Switzerland  
>! [Baarle-Hertog](https://en.wikipedia.org/wiki/Baarle-Nassau) is a Belgian exclave which is an enclave in the Netherlands  
>! [Büsingen am Hochrhein](https://en.wikipedia.org/wiki/B%C3%BCsingen_am_Hochrhein) is a German exclave which is an enclave in Switzerland  
>! [Nahwa](https://en.wikipedia.org/wiki/Nahwa) is an exclave of the United Arab Emirates which is an enclave in Oman  
>! [San'kovo-Medvezh'e](https://en.wikipedia.org/wiki/Sankovo-Medvezhye) is a Russian exclave which is an enclave in Belarus  

Here's more:
>! https://en.wikipedia.org/wiki/List_of_enclaves_and_exclaves