Even though it's really hard to choose I always say The Hitch Hikers Guide To The Galaxy. It ticks all the boxes for me. It's funny, clever, thoughtful, moving. Although I was very young the first time I read it, I'd never read anything like it in my life and it opened doors to other authors that made thinking about the big things enjoyable.
So yeah, that's the one for me.
RIP Douglas Adams.