I drove for 10 days in Eastern Europe, covering about 3,500 miles in my Opel Astra diesel rental car. Went through quite a few countries:
Germany
Austria
Slovenia
Croatia
Bosnia
Serbia
Albania
Macedonia
Kosovo
Bulgaria
Romania
Hungary
Slovakia
Poland
Things were wonderful until I got down to Serbia, Albania, etc. As you transition away from the Alps and into the dry lands with Muslim influence, or more Slavic presence, things get worse. There are cops everywhere pulling people over, the infrastructure looks bombed out, etc. Sometimes the roads are so bad they're nearly undrivable. I program my GPS to avoid highways and tolls, so I was primarily on country roads passing through towns frequently. I never felt unsafe and people were generally very pleasant. I was eyed suspiciously during my time in Albania. Driving north, things improved in Romania. Hungary, Slovakia and Poland are very nice.
Its hard to name a favorite, but I thought the Dalmation coast of Croatia was amazing. I would call this a must-do for everyone.
Next, I am onward to the UK for a week of exploring.