Speaking strictly from a personal perspective, Michael, Brother of Jerry by Jack London is my favorite “classic” novel.
That said, it’s definitely not a book for everyone, so I’d recommend reading a little about it before deciding whether to give it a try.
It resonated with me and my inner state more than any other book.
And I’ll say it again, because I really mean it: this is not a book for everyone. Just from hearing the description, I’ve already had people tell me they would never read anything like it.
For me, though, it put me in a state of mind that was actually good for me.