I think it largely depends on the school and your learning style. I would also preface this by saying don't actually buy anything until you get started and decide how you are going to study and what you need - you may not need any outside resources.
My school uses instructor-written exams (as well as NBME for some blocks). There were some blocks where I found the material in the lectures was sufficient and others where I used quite a few outside resources.
Some of the things I found useful were:
Gray's Anatomy (I preferred the actual text over an atlas, but I also used the Netter atlas to an extent), Costanzo Physiology (I also used Guyton Hall Physio for some topics and review questions), BRS Series (mostly Physio, some Biochem, Gross Anatomy, Neuro, and Path), Robbins (+ Robbins Review), Pathoma, and FA. I thought some of the Dr. Najeeb videos were really useful (particularly for physio and neuroanatomy). I also occasionally used Junqueira's Basic Histology text to be useful on occasion, though I wouldn't recommend actually buying any histology text. I plan on starting USMLE Rx for systems and saving UWorld until a little later this year.
Clearly, I fall on the other side of the spectrum as compared to many of my colleagues. That's probably just because I discovered I retain information better when I see it in multiple iterations from different sources.