Hi!
Depends on what you are looking for and how much time and money do you have. If you're into cities and/or history I recommend visiting most of european capital/major cities and comparing! They are kinda simmilar, all of them has quarter known for pubs, their old town (only Budapes has two old towns haha) etc., but I find the beauty in all these small diferences and wondering where they come from.
Personally I recommend Montenegro and Albania. Albania is cool in every regard and it's hitchhiking paradise. Montenegro has beautiful mountains and some of them aren't full of turists yet. Especially Prokletie mountains.
But hey, my own experiences are very limited.
Best of luck