The "best" textbook depends a bit on the criteria used. For Advanced and MX1, I think Cambridge offers the best variety of questions and with suitably challenging material, but that doesn't make it the best starting place for everyone. Maths in Focus, for example, is overall too simple for fully preparing for exams, but some students may find its gentler level helpful initially - and that's fine, so long as they go on to more difficult material. Similarly, the explanations and theory in Cambridge are clear and concise for some (many?) students, but confusing or insufficiently detailed for some to follow. For a student who finds Cambridge's explanations difficult, I'd suggest looking to other texts to learn theory or address difficulties first.
Ultimately, past papers are essential for effective study and no textbook can replace the need to work on problems across topics and without being grouped as practice of any particular variety.