The absolute best time for visiting this vibrant city is early summer (May-June) and the Fall (September-October). The temperatures are nice and mild at these times.
The transportation is next to none in this city with an excellent public transportation system. One way trips start are only $11. Single tickets are only $2.30. These tickets are good for every single one of the public modes of transportation in Barcelona including the metros, buses, trains and trams. Taxis are also readily accessible in the City at all times.
FYI: Mealtimes in this City are unlike the US. Lunch is usually from 1:30PM-4PM AND 8:30PM-11PM for dinner. An interesting and noteworthy fact is that many storefronts and establishments in the city close from 2-5pm for a siesta, as well as on Sundays and public holidays. Some businesses are even known to close for the entire month of August! These factors are much different from other areas but this region is well worth experiencing with its beautiful scenery and different and eclectic way of life.