I haven’t been to a lot of them but the three favourite ones I have actually spent time in and loved would be Venice Bergen and Budapest. For different reasons.
It didn't occur to me to add a UK city, but I do regard London, having worked and lived there, an extraordinary city, wonderful theatres art galleries, museums, beautiful skyline looking from Waterloo to St Paul's particularly at night and the parks especially Richmond, Bushy and St James's I spent a lot of time in those 3, love them, and other green spaces like Hampstead Heath all combine to make it world class