After travelling in 45 countries and visiting aprox 350 cities you can say that I am an experienced traveller. Most of my journeys is done in Europe, and it is also my favourite continent. My top 5 list here might be a bit different from what you would read in a travelling magazine, and it is because I have some certain critetias I prefere. I do not like when a city is too big, and a perfect size is about 1 million inhabitants - which is basically like a big village.
My favourite city in Europe is Prague. It is also called the golden city, and except being a cheap capitol - it is also a medieveal city which has not been destroyed in wars like others. The river Vlatva is passing through it on its way north, and it holds the biggest castle area in Europe. If you are backpacking around this would be a top notch city to visit because of the price level and the simplicity to get around. It lives around 1,2 million people here, but is visited by as much as 15 million tourists every year.
Porto is a great city on the west coast of the Iberian peninsula. It is a very hilly city situated near the river where they transport the grapes they use to produce the Portvine. Not only is the Portugese people very friendly, but they have good foodtraditions and you should try some of their specialities.
Riga is a nice city located on the coast with the Baltic sea. As with the 2 cities I have mentioned earlier, the arcitechture here is remarkable and it also have one of Europes biggest local markets. A 20 km journey from this city and you will be on the beach in Jurmala. The beachline stretches all the way from Lithuania up to Estonia and is from my point of view one of the best in Europe.
Rijeka is a nice little city in north of Croatia. It is the biggest Port in the country and is not a typical tourist city. Except the friendly people, the city has a lot to offer and it is a short way from Rijeka and to the island Krk. It is, like Porto, a typical student city and has a lively night life.
San Sebastian on the north coast of Spain is a beautiful destination to visit. Visually it can remind you about Rio de Janeiro, but in a much smaller scale. Spain is always a nice country to visit, but I prefere the northern part since it is greener and has nice vinedistricts.
If you have any questions about this destinations, feel free to ask :)