I was disappointed with Casa Vicens, and I've seen just about every building Gaudi designed. There just wasn't as much to see (fewer rooms reconstructed) as at the other sights. The viewing conditions are a lot better; it isn't nearly as popular as the other Gaudi buildings in Barcelona.
Casa Mila/La Pedrera and Casa Batllo are both prime Gaudi. At the time of my 2016 visit, both were extremely crowded (I wondered how the fire marshall felt), but Casa Batllo was a bit worse. It seemed to admit more people in each time block. I have no idea whether that situation still prevails. I'm sure they're both still packing them in.
Palau Guell is an option though not as distinctive as Casa Mila and Casa Batllo. The Palau Guell ticket is probably still easier to acquire. I still wouldn't show up, planning to stand in line, though.
The Sant Pau complex is totally worth visiting (I visited in both 2016 and 2019) but isn't as wackadoodle as the Gaudi buildings. You can, indeed, just walk up and buy a ticket. There are multiple buildings, so it needs a good bit of time.
The Palau de la Musica Catalana is stunning. It's worth walking over there even if you only have time to see the exterior.