Not very good at choosing between cities so apologies for giving you more than you asked for!
Top three:
Istanbul - amazing city, lots of history, doesn’t feel European, great shopping, good vegetarian food
Prague - the prettiest city in Europe imo. The stag parties are a shame but it’s such a romantic city, especially in the winter
Rome - again history, food, culture, shopping!
The rest of my top ten:
Nice - brilliant summer break, blue skies & sea, interesting old town, swimming in the sea in next door Villefranche
Paris - art, food, shopping, culture
Venice - obviously. Best out of season without the crowds.
Stockholm - pricey but attractive and interesting city with lots to see and do
Syracuse - the Ancient Greek part is stunning and there is an incredible amount of history. Good food and doesn’t cost a bomb
Cologne - very quaint, nice place to sit by the river with food and beer
Dublin- the people
Places I wouldn’t bother going back too - Athens, Vienna, Milan