Personally I hated the NXP's docs for the ARM M4 core. Bunch of dry tables listing each register in details, lacking the juicy diagrams and descriptions on how the bits integrate to work as a subsystem. I constantly needed to cross-reference 3+ documents (most of which describe the whole family and not the specific IC). Their HALs and code samples were obviously written by students/interns.

I liked working with Microchip uC, but this was back when the whole IC (PIC24) was described in a single ~1000 page document. I found it very readable and instructive in general.

If I had to pick something today it would be with RP2040/2350. The docs look awesome and there's a huge community that is not locked down in some corporate moderated forum but spread organically, with actually useful GitHub projects. It is the only embedded product where it felt like the open source community is along for the ride and not just picking up the scraps. I hope they continue this line of products.