What are the best places in Europe to visit in Spring?

There's no real answer to this question I'm afraid. Pretty much all of Europe is fabulous in the Spring. To cut things down, and really make the most of Spring (wearing light clothing, enjoying the outdoors), you could maybe focus on areas that are typically miserable during the Winter or uncomfortably hot in the Summer. For early Spring, Southern Europe - Madrid, Rome, Athens - is fantastic. For late Spring, London, Paris, Amsterdam, Berlin have all defrosted and are all gems. You can't really go wrong!