I can get a free return flight ticket to a European city . I'd have to pay my stay and airport taxes and stuff though. And I have to choose from the following cities:

Barcalona
Berlin
Brussels
Geneva
Lisbon
London
Luxembourg
Nice
Riga
Zurich

I've been to all of them except Zurich and Geneva. I visited Barcalona and Riga very briefly, in the other cities I've spent several days if not weeks. But which one would you choose? I obviously can't decide and I don't want to spend too much money .