Legion must be the best expansion ever because it's the first expansion people aren't in the majority of finding it the worst expansion ever.
As for me, Legion is at the top for me. I don't think any expansion was dreadful, but this one is my favorite. That's from someone that's done 7.3 on the PTR however. I'm sure 7.2 has still left people with a sour taste. But it doesn't seem like that feeling will survive 7.3. Unless you're PvPer, perhaps. But they have never been happy.
The people in my guild are having fun. I'm hearing very few complaints. So yeah, I'll happily place this one at the top, above even Wrath.