The best time to visit Barcelona can vary depending on personal preference and what you're looking to experience in the city. However, generally speaking, the best time to visit Barcelona is in the Spring (April-May) and Fall (September-October) when the weather is mild and the crowds are thinner.
In Spring, the weather is warm but not hot, and the city's many parks and gardens are in full bloom, making it a great time to explore the city's green spaces. Also, it's less crowded than in summer and the prices of flights and accommodation are not as high.
In Fall, the weather is still warm and the crowds are thinner, making it a great time to explore the city's many attractions and enjoy the local culture and food. Additionally, fall is a great time for outdoor activities like hiking in the nearby mountains, or for visiting the beaches on the coast.
Summer (June to August) can be very hot in Barcelona, and the crowds are at their peak, so it can be a less comfortable time to visit. Winter (December-February) can be chilly and some of the outdoor activities may be limited due to the weather.
It's important to note that the peak season for tourism in Barcelona is from June to September, so if you want to avoid crowds, it's best to visit outside of this period.