If you're after critical thinking specifically, I'd look beyond what gets labeled philosophy most often. A lot of classics are more about building a worldview than sharpening argumentative tools. You want something that forces you to pick apart assumptions. For that, I keep coming back to 'Thinking, Fast and Slow' by Daniel Kahneman. It’s not pure philosophy, more psychology and behavioral economics, but grappling with cognitive biases is the bedrock of clear thought. It teaches you to question your own intuitive judgments constantly.
Another one that messed with my head in a good way was 'Gödel, Escher, Bach' by Douglas Hofstadter. It's a beast, full of puzzles and loops that make you rethink what it means to think. The connections he draws between math, art, and music are insane. It trains you to see patterns and systems, which is a huge part of critical analysis. Might be a bit of a left-field suggestion, but it absolutely reshaped how I approach complex ideas.