1984, George Orwell. I'll likely re-read this novel periodically for the rest of my life. I'm sure I was impacted by it in essentially the same way most other people have been, so I won't beat a dead horse about this one. That said, I do think I was personally influenced especially by Winston's all-consuming loneliness, and how that loneliness was eventually leveraged against him ("Perhaps one did not want to be loved so much as to be understood"); this aspect of the story was especially horrifying to me and, I think, helped me develop both a healthy skepticism of political social engineering and an appreciation of how important it is to be able to tolerate loneliness (and other unpleasant experiences).
The Phantom of the Opera, Gaston Leroux. I don't think this one had much philosophical influence on me as, frankly, it's not a very well-constructed or consistent novel.
It is a great story, though, and it impacted me tremendously when I read it as a kid and fell in love with it. Phantom in all its various adaptations became a special interest that persisted for many years and resulted in a massive collection, and I still have a deep fondness for it as an adult. Even today, although my interest in Phantom specifically is significantly less intense, its influence can still be found in my love of detective/mystery stories in general (especially those with a gothic, occult, or supernatural twist). By far the most impactful book I've ever read in terms of time, money, and long-term interest.