Take the following advice from someone who has traipsed around every continent on the planet (except one, australia) at one point or another looking and/or smelling horrible. At one point or another in backpacking around Europe, you get very sick and tired of looking hideous while everyone else looks so fantastic. I think the way to go is, map out a route and a time limit you can honestly deal with. Because it's a waste to do parts of europe where you're thinking about how much you need a shower and want to go home. It varies by person. I personally would save the best for last (for my money, Paris), and when I got there, threw away everything I never wanted to see again, bought something new to wear, and hit the town in style (and stayed in a hotel).
That's all I know on that. And after the last olympics, I REALLY want to go to Greece. I think I'm going to try for that this summer.
I have also heard the beaches of Croatia are unmissable. But, if you've never been to the South of France, you should go, and try to get on P Diddy's yacht and see the rest of europe in that thing. Screw backpacking.
Oh, and note, the entire country of France is closed in August.