I don't know if the guide books would consider this to be the crown jewel of the city, but I do. I have been here three times, and I have never braved the line to enter this catherdral, however the outside if almost spectacular enpugh to not see it from the inside. It is a work that was started by Barcelona's famous and unique artist and architect- Gaudi.
He worked on it for the last fifteen years of his life, but he was kind of interesting in terms of creating a large building like this because he didn't like to make plans on paper, her kind of just liked to "go with it" and plan as he went, which has made it and even more challenging project to complete.
You can read more about it on Wikipedia, however please note that it was started being built in 1882 and it not expected to be finished for almost 20 years from now, however some Barcelonians think that is will never be finished, and that for some reason is just always needs to be under construction because it has for so long. Some of the recent issues that have arisen with it have included the implementation of high speed rail in the area and questioning whether or not the structure will hold- some engineers have questioned whether or not the foundation is strong enough to support a structure such as this regardless of the introduction of high speed rail vibrations in the immediate vicinity. It would be a tragedy is this thing collapsed, so here's to hoping for rgood engineering in this case.