In May I visited Barcelona and visited Palau Guell. This was the first Gaudi building we visited and it didn't disappoint. This was once a personal residence. It was built between 1886-1888. If you are a fan or architecture and design, I highly recommend going inside Palau Guell. It wasn't very busy when we visited, giving us lots of time to wander without feeling like we were moving with the crowds.
For more info check out their site: https://inici.palauguell.cat/en/