Ohhhh man, this is definitely one of those "the chicken or the egg" questions ahaha.
I think in an ideal world, the answer would be both -- interesting plot written beautifully.
In the real world, however, I will take an interesting story with flawed writing any day over bland purple prose. Especially after the 9-to-5 grind of adulting, at the end of a long day, I won't have the brainpower to whip out the dictionary every other sentence just to understand a 3-paragraph description of...a door, or something.
I read one such (traditionally published) book a while ago, which was absolutely beautifully written. Stunning. The characters and story, on the other hand...were ultimately disappointing.
Conversely, last summer I binged an (online published) teen fiction book which I normally am not attracted to. It was so funny, and written with so much heart, and even though the writing wasn't extraordinary or perfect, I enjoyed it immensely.
So, I would say, a fair balance of decent writing and gripping story is the best combination!