Probably Initiation into Hermetics (IIH). I will admit that I am a bit of a Franz Bardon shill in some ways, but IIH was an eye opener for me in some ways. Compared to a lot of Golden Dawn, Crowley, and a lot of other western occult works, IIH really provides a unique perspective on the elements and the "Hermetic" world in general. The exercises are tedious and I only loosely follow them, but they are powerful when used correctly. The most valuable part is seeing how the meditative, concentration, and energetic exercises can connect almost seamlessly with other occult systems of practice. The perspective is written from western mystery tradition with a somewhat more profound understanding of eastern spirituality mixed in (at least compared to other authors of the time), which makes it somewhat syncretic.
The theory and exercises transformed the way I do approach magic and gave me the motivation to start going out of my usual comfort zones such as GoM and traditional Goetic grimoire work. I have used its concepts even in play testing some Arabic magic from Nineveh Shadrach's work. The perspective and understanding IIH equips you with can easily be translated to other lesser known systems if you are willing to integrate them. I think as a blueprint and a roadmap for magic, IIH is second to none. But it is not without its flaws of course.
I never believed that Bardon is some sort of saint or that this work is a true gift from "Divine Providence", but the guy was nice enough to write us an excellent training manual on magic that is extremely practical. Many people think he only borrowed from a limited number of sources, but over time I realised that the scope of Bardon's sources is impressively large. That scope means that IIH is a work that is informed by a wide range of influences and for the most part it is surprisingly coherent and workable. This is quite rare for a "manual" from those days. And while I do respect Crowley and GD works, a lot of their stuff is far too symbolic and a bit self-aggrandising. IIH and Bardon's training is a lot more humble and focused on the idea of "you need to earn your keep" without using official grades and ceremony. For me, this is something I enjoy even if Bardon is a bit dogmatic on some parts.
TLDR - IIH really unified my understanding and practice of magic. It gave me a perspective that allows me to look into other magical systems more deeply and its own exercises are a powerful supplement to magical practice. The scope of the book is larger than expected even if it is smaller, but it is surprisingly profound in its contents. It is a powerful addition to anyone's magical training if they feel a vibe for it.