https://en.wikipedia.org/wiki/Fear_and_Loathing_in_Las_Vegas...
That passage remains one of the most meaningful things I've read in my life.
However, his defining work in my opinion is Hell's Angels. It is prescient in so many ways. Among the most interesting is how early he is able to see, inside the Hell's Angels motorcycle gang, the rise of so many later developments in US politics. I will not ruin the surprise, but some of the passages would make you swear that the man had a time machine. I've read it 3 times now.