Rembrandt - Portrait of Hendrickje at the Window
Significant painters can be significant in many different ways. A painter can give great pleasure those who look at his work. He can have great influence on his colleagues and successors. He can do something in the realm of theoretical aesthetics which has never been done before. Or he can solve some great technical problem.
It's not a good idea to confuse these different ways of being significant. Especially, it's not a good idea to assume that being very significant in any one of them makes a painter great.

There's no reason to think that Rembrandt knew more about depicting beauty than anyone else. This portrait of Hendrickje, like the others of her, makes one tend to think that he didn't. Just compare Rembrandt's paintings of his mistress to Renoir's paintings of his servants, or incidental young girls. Neither is there amazing in the composition here.

Rembrandt was good at portraying textures. But not obviously better than many of his colleagues in Holland. If he has a claim to fame, it is in the technique of representing the illumination of dark objects from a single direction. He also knew how to use this technique to intensify emotional effects. And this use of light influenced generations of students and imitators.

The question is whether these accomplishments justify his enormous reputation.

Spaces, not commas, please!