Most important text I read and that I read continually: Being and Time by Martin Heidegger, arguably the most important work of non-fiction in the 20th century. All philosophy begins here. His entire corpus is included in my list, but you must start with B&T. No Heidegger, no Orthodoxy for me.
We briefly read some Heidegger in my Contemporary Philosophy class this last summer, and we will read some again in my Phenomenology class this spring. Maybe at some point I'll read all of Being and Time so that I can understand why this particular philosopher had such a profound impact on you. I look forward to it.
It showed me, like every other thing that mattered, was that most the questions I cared about were the wrong ones and how I gotten to those wrong questions in the first place and showed me where to begin trying to think.
Frankly, as much as Heidegger would strangle me, you can get similar results from Freud and perhaps Marx. And if you have a truly sharp mind which can span the so-called analytical to the synthetic, then Goedel can do it as well.
But I've yet to see anyone frame the problematic of metaphysics so eloquently and straightforwardly and concretely as Heidegger.
If you have the right sense of humor, neech can do it as well, but I've met maybe three people ever who did. If you ain't laughing when you read neech, you are doing it wrong. Most of the time with Heidegger as well. Maybe less of a laugh and more of a grin.
Favorites are stupid, but I will say that really in terms of impact nothing probably was more singularly altering my life than the Euclid we did in 9th grade. Just some dumb poor kids working though worn out abridged versions of
The Elements. That is what really cast the die for me. It allowed me a clear grasp of my potency for the metaphysical prejudice of our time and then led me to Goedel via mathematics / logic which lead to the dead end which opened with Heidegger.
It was weird that in that stupid school I had a math teacher who was actually a PhD, who for reasons too long to mention, worked in a no-man's land of education. I didn't realize till we had an exchange student from Turkey, who was light years beyond us in terms of education, how accomplished our teacher was and how generously he allowed some us to take from his experience.
If it wasn't for him, I wouldn't have had the chance to able blow many awesome opportunities.