La Sagrada Familia is one of Gaudi's most famous works in Barcelona. It's a giant temple that has been under construction since 1882 (that's not a typing error!) and it's not expected to be completed for between 30 to 80 years ( time for completion varies depending on different sources of information)
(from the website: http://www.barcelona-tourist-guide.com/en/gaudi/sagrada-familia.html)