You have to examine overall trends in Eastern Europe to come up with a property matrix; given de-population, brain drain and ongoing urbanization, it means urban areas and coastal areas can be good ideas, but you're already behind trend in many of these places given massive influx of Western Tourists and investors to Eastern Europe. For example, the Montenegro ship has already sailed, and as the East continues to improve economically opportunities will only disappear, as well as the fact the EU continues to expand ever-eastward... Easy Jet flights seem to be going almost everywhere these days.
In reference to Bulgaria, which another poster mentioned: Bulgaria is a definite opportunity, but the ownership of property is largely confined to tourist locations and larger cities; like Sunny Beach, Varna and others. Bulgaria is hugely depopulating and if for example you wanted a property in a smaller town or Tier 2 city, you would have your choice of deals.
In Poland, Poles continue to return to the country with their high wages from Western locales, so I only see Poland improving more in the future. Poland would of been an awesome opportunity about 10 or so years ago.
Other places, like czechia, romania and hungary seem to be on everyone's radar screens.
Croatia too, would be an awesome place to invest in, but its a lot like montenegro given the recent explosion in property prices.
I'm of the firm belief that eastern germany is a value, relatively speaking to most of the rest of western europe given the economic disparity between east and west of germany.
Especially in the East (of europe) it can be harder to own property if a foreigner, the obvious way to get around this is a locally based company which allows you to circumvent these restrictions.
In some places, Balkans and other parts of eastern europe, state enterprises are still in the process of being privatized; so if you can play this game there are some real deals.
I take the pessimistic view on Ukraine, it is massively f***ed and given how screwed up the situation is there, I don't see much changing in the coming years.
But, we're leaving out Europe... some parts. Georgia, for example. Given recent positive press and increased tourism there, I think Georgia is becoming a great idea. Very pro-west country, increasing value of passport, lots of good reviews. Awesome food, wine, etc.