If you were told it was the best Star Wars movie you may have built up an unreasonable expectation. To be fair, Empire is a bit annoying in its ending- although Han’s disappearance is great for the trilogy, as an individual film it isn’t a strong conclusion. I could see how one could be disappointed.
The same thing happened to me with Pulp Fiction, after being introduced to Tarantino with Reservior Dogs (which I like very much) I was surprised to find I didn’t like Pulp Fiction, the legendary movie everyone was hyping me up for.