I read primarily non-fiction but in search of the kinds of aesthetic, literary, stylistic merits which you seem to associate with fiction and poetry. (For the record, I also have a lit degree, just in case that gives the following any more authority than it would carry otherwise.) As such, here are a few writers of nonfiction I consider worth a look:
Orwell has already been mentioned, and rightly so. Although he's primarily known for his fiction (especially the Big Two), to me he excels as an essayist and writer of nonfiction. If Politics and the English Language hadn't already been linked to, I'd be linking it here. He has plenty of other classic essays, though, and his book-length nonfiction is great too. My favourite of these is Homage to Catalonia, which is about as rigorously clear and honest a bit of political reportage as I can conceive of, not to mention being an account of a really interesting part of history (the Spanish civil war).
Borges also springs to mind. Penguin publish a wonderful selection of his essays, and it's absolutely one of my favourite books - easily as good as his fiction. (I mention his nonfiction since I'm on a roll with that, but really, his fiction is just as stylistically great. Plus, reading Borges' stories is a mind-expanding and beautiful experience that everyone should go through anyway.) He has this wonderfully understated and laconic prose style, and his subordinate clauses can express ideas that other writers would belabour for a paragraph. His essays cover an unbelievably broad range of topics, discussing, for example, Zeno's paradox, Citizen Kane, artificial languages, and many, many, many other writers. (He's so well-read in world literature, and interpolates so much from other writers, that one feels well-read by proxy after being in contact with him.)
I'm a great fan of Douglas Hofstadter, for his linguistic whimsy, puns, form-vs-content gags, and other surface details, but it's possible that these might not be everyone's cup of tea.
I've also been digging Montaigne lately, whose book Essays is responsible for the noun essay meaning what it does. His style, though, is quite flowery and circumlocutory, interpolating references to a lot of latin and greek. If you're looking for concision and directness of thought, you probably won't find it in him. The cultural gulf of all those centuries passing also makes him quite tricky to read. But on the whole I was surprised by how much I felt I had in common with him, and there's something deeply heartening about finding that across such a deep gulf of time. And he is an important pre-enlightenment source of skepticism, which might be of interest to LW types.
Last but not least, I've always held Bertie Russell to have a nice clear, direct style.
If I had to pick out one writer from the above, it would be Borges, with no question.
I'd also note that there is a fairly good cached answer to the question of who the best writers in history are, in the form of the literary canon. You'd do well not to ignore it. Literary types might be quite different to LW types, but their implicit consensus about which writers of the past are worth paying attention to is basically trustworthy, I think. In other words, reading pretty much any 'classic' (roughly speaking, 'well-known work from before the twentieth century or so') is likely to expand your horizons and teach you something new in terms of style. On the other hand, changes over time in language and culture form barriers to easily reading sufficiently old books, and I suppose one could argue that this outweighs what one stands to gain from it.
Borges also springs to mind. Penguin publish a wonderful selection of his essays, and it's absolutely one of my favourite books - easily as good as his fiction.
Borges is also my favorite top author, and I also think his nonfiction is at least as good as his fiction, which hardly anyone else does! Were we separated at birth?
We currently have a thread in progress concerning the greatest philosophers in history. This reminded me of a question I've been considering for a while.
Recently I realized that although I've spent a massive percentage of my time for the past few years reading, it's almost always been non-fiction, for example posts on Less Wrong, or posts on other forums or blogs, or old treatises on economics or philosophy. Although I feel as if I've stumbled onto some of the absolute best sources of insight the history of civilization has to offer, and some of the most brilliant thinkers to have ever walked the planet, it strikes me that the main optimization criteria that led to the popularity of most of these pieces of writing and most of these thinkers, and thus the reason they were visible enough for me to run into them, has been a matter not necessarily of the quality of writing style, but instead a matter of the level of insight perceived by whatever critical mass of people brought it to the forefront.
In other words, I've mostly ran into articles and books that have garnered their level of fame not by the eloquence of their prose, but instead by the insight contained within. I've ran into plenty of famous, highly insightful thinkers who just aren't very good at writing. They became famous for another reason: the ideas they brought to the table, however incompetently. This is in utter contrast to another section of the history of writing: what we call "fiction", and in some cases "poetry". Although fiction writers or poets are generally expected to bring some sort of insight to the table, or even a lot of insight, this certainly isn't the overwhelming criterion determining their fame. Most of the scientists who have become famous for their work have been at least decent at writing, or nobody would have been able to get through their stuff (though there are exceptions). In the same way, the great fiction books of history certainly have insight; it's just often not what carries them to success.
In non-fiction, the top contenders are usually there for their insight, though their writing is usually at least decent. And in fiction and poetry, the top contenders are usually there for their eloquence and writing style, though their insight is usually at least decent. There's at least one exception I can think of, where this person seems to be civilization class in both insight and writing style: David Hume. One of the greatest thinkers in history, and certainly also one of the greatest writers in history. Anybody who's read a decent amount of other famous writers and thinkers, and understands some of Hume's key arguments, would at least have some sort of sympathy for that characterization, despite the extreme level of praise I'm bestowing onto his work.
So here's my point, and my question: I'm mostly interested in insight, but I'm also interested in communicating this insight in an extremely effective way, with the most solid prose possible, and the highest level of eloquence that can be attained. For this, I can't simply limit myself to reading the most insightful, revolutionary non-fiction work, as the market test that led to the widespread adoption of this work was not necessarily one of requiring a high level of eloquence or poetic ability. I'll need to read works that became famous for their writing style. So we come now to the question: Who are some of the best writers in the history of civilization? Who should I read for the purpose of getting better at writing?
(TL;DR: I've spent a lot of time reading non-fiction books, but most of these books became famous for their level of insight, and not necessarily because of any sort of high level of competence in writing, or eloquence of style. I want to get better at writing, so although I've ignored fiction for a long time due to thinking my interest was scientific insight and not fiction stories, I've changed my mind, and realized I should probably be reading some fiction in order to learn from the masters of eloquence. So with that said, here's my question: Who are some of the best writers in history?)