I don't think there's one good answer.
When Stellaris was released, we had few space 4X, especially ones that tried to reinvent the genre. There was Endless Space 1, which had some great qualities (interface, music, empire design) but still a lot of limitations. Stellaris came with dynamic storytelling, dynamic empires (they even used to change names when they changed ethics!), mixing new ideas with already known concepts in exciting ways.
We tend to forget that Stellaris was released with a lot of features that just created something completely new that revived the whole genre. The slaver freedom faction, Fallen Empires, Primitives Civs, FTL types, Tile-based planetary management, ethics and governement systems may look trivial or even obsolete today, but when Stellaris was released it was massive.
It's true that some people did complain about the lack of content and balance at release, but you could already feel the potential. It's quite funny btw, we were a lot to think that factions and planet types were going to be a big deal, but that's not really what happened.
Of course different people started to play at different times, and started to enjoy Stellaris at different times. Personally I have fond memories in most updates. Actually, I was quite disappointed in leviathans and the associated update, but my worst memory is probably the AI empire starvation bug. I can't remember when exactly it happened, but I remember that it happened together with a huge Quality Assurance crisis. Issues like that were starting to pile up, until they finally created the Custodians team.
So if I had to pinpoint when Stellaris became so good, I'd probably say after the creation of the Custodians, simply because it improved the game so much on the long term. But Stellaris just gradually became better along the years.