For students of both undergraduate and graduate level, if there is a Griffiths textbook for the topic you are currently investigating, that is what I would recommend. For undergraduates, Griffiths provides a wonderful introduction to each subject. His books contain adequate theory and mathematical principles, numerous examples, and an insane number of useful problems to work through to ensure a students grasp of the material(as well as a good dashing of humor). I actually have a habit of placing a blue sticky note in a book when I find an illuminating example or problem that can be essential to understanding a concept, and I often find Griffiths texts to be a sea of these blue notes by the time I am finished. I also recommend Griffiths for graduate students, because from my experience if you can understand a majority of the Griffiths text for that subject, it really is trivial to apply graduate concepts to the material(as simple as adding cheese to a burger), and qualifying exams and problems that will be encountered in your research can often be tackled with the depth of the topic that is covered in Griffiths.
Of course aside from physics, there are plenty of textbooks (particularly mathematics) that I find particularly illuminating, and I will bring these up when they are necessary or helpful for the subject that is being tackled.