What do you like doing and when will you be going? Maybe combine a visit to a country with a festival - music, cultural, beer - or maybe do a multi-country hop and enjoy the best of a couple of places.
If you are looking for a party then capital cities could be your best bet, or head out to somewhere like Ibiza which is known for its fun and hedonistic life. There are also much quieter (and nicer) parts to Ibiza.
Berlin, Stockholm, Prague, Barcelona are all great cities. about 6 years ago