Just returned from Croatia, Slovenia and Serbia. Needless to say I poured all sorts down my throat (beers of course).
Croatia - by far the best was a beer called Ozujsko, its the most popular, the locals like it and it tastes good. Tried some called Karlovacko which wasn't a bad drop and also some called Pan, which is essentially Croatian Carlsberg!
Slovenia - Lasko or Union are the main beers of choice. Union beer allllllllllllll day long, lovely drop!
Serbia - without wishing to offend any Serbs we have here, I didn't enjoy the country at all, but the one good thing I found was their Jelen beer, it was a cracking pint and if you do go to Serbia then I suggest you have a go.