Frank Hardy's Power Without Glory is first on my list, but the rest is mainly science fiction, for instance Isaac Asimov's I, Robot which was once of the first Sci-Fi books I ever read. Though I think that was preceded by a couple of Jules Verne's.
Some of those that made a great impression were maybe not great writing, but they just came up with such fascinating concepts. For instance, A. E. Van Vogt's little novel about the God-king of Gondwanaland, forget the actual name of the novel. ("World of..." something or other?) But the initial concept of a vast population that can make a man (and/or woman) into an immortal God merely by worshipping them (through some mechanism or other I vaguely recall, but that's just a detail) just made a lasting impression. You can't help wondering if the writer's deliberately taking the piss out society, or just by accident.
Or Niven and Pournelle's Burning City (which is labelled Fantasy, but I usually hate fantasy novels.) That was about a people and their God too, only their God was a God who regularly destroyed them. Just so alien it almost shocked me. Never got around to the sequel, but the sequels from those authors are always bitterly disappointing. (Ringworld Engineers being maybe the exception, balanced by the sheer awfulness of the third book in the Ringworld series. Now they are selling yet another sequel, not to this little black duck I can promise you.)
Lots more of the SF genre made a great impression. More than I could possibly list. There's a couple of other obvious early ones which I can't even think of, either the authors' names or the titles. All very well to say look at the bookshelf, but even when I do keep a book, they just seem to disappear.
Sci-Fi still keeps me interested, though I've found lately that some of the writers I likes bore or even irritate me now. But there's new ones I discover every now and again, like Iain M Banks.
I'll toss in an honorary mention to Colin Wilson for his Spider World series, though I'm not sure why exactly. All a bit far-fetched and some of the details are a bit irritating, but he nearly pulls it off and deserves credit for that, given the scale of the task.
An odd novel of a different genre (or is it?) that fascinated me was a huge novel written in the 19th century by Eugene Sue, The History Of The People. Its hard to beat really good historical fiction (often a great excuse to read a good war story) and The History Of The People, a fictional story, or series of stories, spanning two thousand years of history, makes other historical fiction pale into insignificance.
Speaking of historical fiction, I suppose I should probably include Solzhenitzen's First Circle. Though i guess it was actually contemporary when it was written. But I only read it fairly recently. ;-) That was really fascinating, chiefly for the analysis of the characters. Speaking of which, Orwell's Burmese Days and the Spanish Civil War diary can't be left out. What the hell, I can't remember anything of Orwell's that wasn't thought-provoking. But that elephant story of his, for instance, that was deep.
Speaking of deep, then there was my Western phase, I've been on the wagon for awhile so I'd better not dwell on it else I might lapse. To avoid embarrassing myself, and of course to avoid corrupting the young and impressionable, I won't recall my favourites.
I better plug a Tasmanian Author I suppose. James McQueen's The Floor Of Heaven will have to do. (Only one can think of, truth be told, but I'm sure there must have been others.) Has it all for a Tasmanian novel, a humorous take on Australian working class culture, incest of course, the nitty-gritty of pioneer spirit and endeavour, convicts even. Really delves into some of the skeletons in the closet, the ones that made delving into family history a big local taboo until recently.
But Power Without Glory stands out for me, even after all these years. You can't get any more hard-hitting than this. When it was written the main subject of the novel used his influence to have Hardy prosecuted for criminal libel. That's not civil libel, actual CRIMINAL libel. Unheard of before or since. Hardy was found not guilty in the end, but he took a big risk. (Hardy had to typeset, print and even distribute it covertly, the story of the publication and the reaction is better than many novels in and of itself.) But it is far from a dry work. It is a brilliant and gripping drama of crime, corruption, Labour politics, working class life and early twentieth century life. A real-life Australian Godfather novel. About thirty years ago the ABC made it into a TV drama series, which was not bad I recall. There was a fuss about that too, with the descendants of the main character doing what they could to counter it and the series has never seen the light of day since. There's probably another story there.
Just read it. I promise you won't be able to put it down.
Bill Bartlett Bracknell Tas