Don't forget casa Batlló - visits are by appointment only (I think) but even just seeing the building’s exterior is pretty awesome.
Battló is the best one for me by far. I love the organic shapes and the light well and the soft wood etc. Wow
Today you really have to reserve, sometimes months in advance if it’s during the high season.
Today I wouldn't even dream of that. You really have to plan ahead for all the major sights that include an entrance.