It's all a matter of personal opinion, but the way that I rank games is by how much fun or enjoyment I derive from them, and according to that scale, TW3 main game was just as good as the expansions.
What I liked about
Wild Hunt is that it truly made me feel like I was living the life of a Witcher, and the personal story of a man trying to come to terms with his past and reclaim his family was more appealing to me than a traditional war narrative, which I feel would have been the easier and more cliche route to take.
The open-world form - as ubiquitous as it may seem - is still in its infancy and undergoing growing pains.
Wild Hunt had a number of flaws, but as an experimental game in a nascent genre, I think it's successes outweighed its failures.
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It wasn't like with Eredin, when it was basically "he evil, no why, just kill'em" ...
While CDPR has admitted that the central antagonist of Wild Hunt could use additional background, I think the impetus for killing Eredin was made pretty clear throughout the course of the game. He's trying to abduct Ciri, use her for his personal ends - i.e.creating a portal between Tir na Lia to invade Geralt's homeworld, escape the White Frost, and enslave humanity - and then discard her. When I thought about his character and the predicament of the Aen Elle, I actually found him to be a somewhat empathetic villain, since he is really just trying to save his people, albeit using morally dubious means. In any case, I construed the main campaign to be more about simulating the day-to-day life of a Witcher, and shedding some light on the familial bond between Geralt and Ciri, something which was heretofore unexplored. In that regard I believe that it succeeded remarkably.
Also no teases or half baked ideas, like sorceress lodge.
There are lots of "half baked ideas" in the expansions, particularly
Blood and Wine. Syanna barely features in the DLC, and when she does, it's as part of a ham-fisted romance with Geralt. There's also the reoccurring issue of lack of reactivity to player choice, e.g. if you make the decision to save Syanna, Anna Henrietta grants you an award for your valor, but there is no follow-up dialogue which comments on the status of her relationship with her sister. Not to mention the fact that the decisions which go into determining who visits you at Corvo Bianco make zero sense.
I'm not attacking
Blood and Wine because I disliked it - on the contrary, I loved it just as much as the base game - I'm merely trying to show that the expansions themselves are not free from flaws, and that it's easy to forget that they were built on the foundation - both technical and artistic - which
Wild Hunt laid. There could have been no
Hearts of Stone or
Blood and Wine without the previous efforts that went into making the main game.
With expansions, they actually didn't try to make fanfic cannon like in MQ and instead did their own thing, which worked great.
Blood and Wine is arguably even more permeated with "fan service" than
Wild Hunt. Anna Henrietta, Regis, they are all characters from Sapkowski's novels. Regis in particular was another person that CDPR literally brought back from the dead in order to placate fans. I loved seeing him, and his inclusion was a stroke of genius, but your statement is factually incorrect.
But truly, I like main game and expansions. There is no best one.