I can confidently say that the best order depends on what kind of literary journey you're after. If you want to start with his most accessible and emotionally gripping work, 'Stoner' is the perfect entry point. It's a quiet masterpiece about an ordinary man's life, and its simplicity makes it a great introduction to Williams' style.
For those who prefer historical depth, 'Butcher's Crossing' is a stark and brutal Western that explores obsession and survival. It's darker than 'Stoner' but equally compelling. After these two, 'Augustus' offers a grander scale, blending historical fiction with epistolary storytelling. It’s his most ambitious novel and benefits from having a grasp of his earlier works. Save 'Nothing but the Night' for last—it’s his earliest and least polished, but interesting for completists.