To a certain extent, it depends on what your interests are, and what you like to see and do on holiday.
Croatia is a stunningly beautiful country - the entire Adriatic coast is spectacular and well worth taking several days to travel along. I was there on a good number of occasions in the years immediately after the recent Yugoslav wars, and so, tourism, at that time, was only beginning to recover.
The east of the country is less beautiful, and also, less historically interesting; for those with a sort of perverse interest in "war tourism", Vukovar can be rather unnerving.
Zagreb is a very nice city - but, frankly, not a patch on Belgrade, which makes it seem what it was, a regional capital.
For my money, I'd start with Pula, up in Istria, the Italianate influenced north east of the country, just across the Adriatic and work down the coast. The scenery, food, architecture are all Venetian influenced, (Venice ruled this region for several centuries), and before that, the legacy of Rome is still evident.
Anyway, Pula has a nice harbour, some amazing Roman arches, and one of the finest extant Roman amphitheatres still standing. It was simply spectacular. I visited it in January 2000 (I've observed several elections in the country), and they opened the amphitheatre specially for us. It was an amazing experience to be able to walk on the sand, and admire the perfect acoustics and flawless sight-lines of a building constructed in the days of the Roman Republic.
Split - about halfway down the coast - is incredible, and the old town is really beautiful. Diocletian's Palace - actually, the old town, an amazing warren of streets, is inside Diocletian's Palace - is spectacular, and the views from the harbour exquisite.
Further down, you have elegant old towns such as Makarska (where the fruit brandies of the region come from), and of course, the peerless Dubrovnik.
Dubrovnik is well worth visiting, but, as DP has already pointed out, Bosnia's tiny coastline intrudes on the way there if you are travelling south - necessitating two border crossings. The food is excellent, as the the wine.
Island hopping is also recommended, as some of the islands are really beautiful and well worth a visit.
You'll have a great time as it is a truly beautiful country; enjoy your trip.