That depends on what kind of tourist you are. If you love the heat, want to see lots of people in the streets, lie on the grass in the parks, sit on the sidewalks in the cafes to watch the movement, the best time is summer.
If you want to enjoy an extra cold that is not available in your city, come in winter.
For those who like milder temperatures and the joyful atmosphere of the people because winter is over, spring is perfect. You can see the wonderful colors of the flowers and see the branches of the trees that were bare, fill with leaves again.
And to see the wonderful colors of autumn, I mean breathtaking landscapes, come between October and November.