Although I have read many textbooks, I have not read too many textbooks for each subject, so it is quite difficult to write a list of "best textbooks" for each subject. Time is limited. So I have to make the best out of whichever textbooks I do get the time to read. Having said that, these are some of the textbooks that have had significant influence on me and broadened my understanding of the abstract or concrete world around me.

1) Computer System Architecture (3rd ed.): This book was phenomenal in my life because it taught me exactly where the hardware meets the software. This book helped me to understand exactly how the hardware fetches, decodes and executes the software on a physical electronic circuit. In fact, I liked this book so much that I implemented the theoretical machine described in the book (called Mano Machine by many) on a Xilinx XC9572 PC84 CPLD Trainer kit. I did not make any assembler or compiler for it, so I had to push DIP switches on the kit to load instructions on a RAM (implemented on another kit) to program the CPU and test it.

2) The C Programming Language (2nd ed.) by Brian Kernighan and Dennis Ritchie: This is the first programming language book that I read cover to cover. It was quite formative in my journey of learning to program the computer.

3) Introduction to Analytic Number Theory by Tom M. Apostol: I found this to be a fascinating textbook on analytic number theory. The book begins with simple properties of divisbility but it then soon introduces several new concepts like the Möbius function, Dirichlet product, the prime number theorem, etc. The book exposes various subtle nuances of the Riemann zeta function with great rigour and thoroughness. A complete analytic proof of the prime number theorem is also presented in this book.

4) Concrete Mathematics: A Foundation for Computer Science (2nd ed.) by Ronald Graham, Donald Knuth, and Oren Patashnik: Great collection of problems that are insightful and entertaining.

5) Gödel, Escher, Bach: an Eternal Golden Braid by Douglas Hofstadter: Not exactly a textbook but it is a very intriguing book with a large assortment of interesting concepts related to formal systems, paradoxes, self-reference, isomorphism, meaning of "meaning", etc. all packaged neatly into a single book.

Would like to answer but the question is too broad. What subjects are you talking about? Programming, algorithmic, logic, hacking, and others linked to informatics in general, or other subjects? And by textbooks, do you mean out of all the textbooks in general, or per difficulty level (ie introductory, advanced, etc.)