What are the best cities in Europe to visit?

I will be travelling to Europe after graduation (summer 2011) with two other friends. We have compiled a list of possibilities which we need to narrow down to between 6-12 cities. We plan to spend at least fifty days there altogether. I would like to hear the opinion of someone with first-hand experience, and your... show more I will be travelling to Europe after graduation (summer 2011) with two other friends. We have compiled a list of possibilities which we need to narrow down to between 6-12 cities. We plan to spend at least fifty days there altogether. I would like to hear the opinion of someone with first-hand experience, and your suggestions on which would be the best for a first-time visit - also if there is any other information or tips that you can provide, that would be excellent...thanks in advance.

LONDON
ATHENS
ISTANBUL
CAIRO
FLORENCE
ROME
EDINBURGH
MADRID
ROTTERDAM
PRAGUE
BRUGES
ST PETERSBURG
DUBLIN
LUXEMBOURG
SALZBURG
MUNICH
SEVILLE
BUDAPEST
STOCKHOLM
OSLO
SANTORINI
DRESDEN
BARCELONA
VIENNA
MOSCOW
VENICE
PARIS
KRAKOW
BRUSSELS
AMSTERDAM
ZURICH
DUBROVNIK
BERLIN
LISBON
HELSINKI
Update: Yes, thank you, I am well aware that Cairo is not in Europe. Istanbul, however, is.
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