u/Dal-lyone

Serbia, but they managed to snag Albania during the confusion in the 1920s, thus leading to the current predicament it's in with Albania being an autonomous region. It hasn't broken away yet, but recent referendum numbers look increasingly like Albanian independence is finally going to be won.

As for why Serbia holds, North Macedonia, Montenegro, and a portion of Bosnia as well as Croatia is simply because those regions specifically voted not to separate from Serbia during the Yugoslav Referendum (1990).