It's out of the question, everyone will vote for athens, it's a popular tourist destination. I, as a BUlgarian felt obliged to vote for Sofja. The question here is more like Sofia or Bucharest. Well, I'm a little prejudiced since I live in Sofia, but IMO:
Sofia is similar to Bucharest in the sence of maintenance, they are both not exactly clean. read uninviting. as for architecture it is closer to belgrade. bucharest could be considered more interesting as it has a huge collection of megalomaniac stalinist buildings (so megalomaniac they were finnished in the 80's), but sofia has Vitosha, a wonderful mountain right near it, so it's more of a choice for nature-lovers. It's terribly underrated and unknown. so here are some pics
http://www.pbase.com/ngruev/sofia