You should see Paris, it's one of the most beautiful city's in western europe. I've rarely seen anything as beautiful as the sacre coeur. I'm personally not a huge fan of southern europe. Sure, the weather is good and the beaches are nice but I find them to be quite boring. Considering that you're going in the winter, I wouldn't waste too much time on the shores since most of the towns will be like ghost towns with absolutely nothing to do. The capitals are still very nice, but if I was you I'd skip greece, from what I've heard Athens isn't all that great. Greece is a nice country though (been there twice), but you should visit greece by island hopping in the summer. I've never been to Spain since it doesn't really appeal to me at all, but Barcelona and Madrid are great from what I've heard.
If I was you I wouldn't want to miss out on eastern europe. The capitals are really amazing, like Prague and Budapest. It's filled with beautiful historical sights, museums and it has a great night life. Apart from that, it's also very cheap.
You can't miss out on Amsterdam and London but I don't know if you can fit that all into your holiday. But you should see those city's at least once in your life.
Scandinavia is beautiful but in the winter it's very very cold and it's dark most of the time. The capitals of Norway and Sweden are also among the most expensive cities in Europe.







I'm not a huge fan of Italy, especially northern Italy. There's absolutely nothing to do and people close their business for like 3 hours in the middle of the day for nap time




