João Diniz (.
Google
Worth visiting, I had no waiting time. There’s a very short exhibition on the lower level but that can be skipped - it’s all about the 360 view from above, 30th floor.
Fantastic to get an idea of the city, and there are some indications/notes on what can be seen, such as the main attractions & reference points in the city.
Not particularly cheap though, but with the red tourist Barcelona card I only paid €14 p/person (without that it would’ve been slightly over €20 I believe).
The key point is to try it out on a sunny, cloud-free day for the best views and pictures. No need to book in advance - if time allows, I’d reserve 1-2hrs on the clearest day for this.