The Sagrada Familia, is a magnificent church, designed by Gaudi. It is not yet complete, and has been in progress for over one hundred years. I hope that I live long enough to see it complete, but even in it's current state, it is a must see if you go to Barcelona.
Most churches and cathedrals have an inherent stuffy feeling to them. Gaudi, has built a church that looks as though it was built by the heavens and earth. The exterior looks organic, as though a forest had decided against random chaos, and all teamed together to make something magnificent.
The interior is bright and cheerful, the high ceilings, and the superb use of stained glass give the interior a ethereal, heavenly feel. Instead of created images with the stained glass, he has themed the windows by color, this creates stunning swatches of color within the church.