I'm from Latvia and have been to:
Lithuania (like 3 or 4 times 'cuz it's right next door, possibly more, mainly Klaipeda, Palanga & Vilnius)
Sweden (only Stockholm thou, but 3 times 'cuz it's just a ship away)
Poland (Gdansk and some other places)
France (Strasbourg)
UK (London)
Czech Republic (Prague and a bit of country)
Hungary (mainly Budapest)
Croatia (I think I've been all over that one, but mainly by the coast)
Malta (mainly Valletta)
Madeira (went all around it, so, nearly everything)
Spain (Madrid & Salamanca)
Portugal (lived there for 5 months while on Erasmus soooo - saw quite a lot, but mainly lived in Barcelos then Porto & a bit in Lisbon)
I think that's it, haven't been anywhere else, 'cuz countries, like Germany or Slovakia, Austria, Slovenia etc while I have been there, it was only while traveling through, so they don't count.
My favorite is probably London (because I got to see all those places we see in movies in real life) & Croatia, Malta, Madeira 'cuz they have the kind of nature we don't have here - warm (hot) weather, real mountains & azure water ^^