Most of Sony's smartphones start out far too expensive, but slowly drop in price until they become reasonable purchases. Last year's Xperia 1 is an excellent device if you don't mind having the tallest phone around, especially now that it's only $598 from Amazon.
The Xperia 1 has a Snapdragon 855 processor, 6GB of RAM, 128GB of internal storage (with microSD support), a 6.4-inch 4K OLED screen, a 3,330mAh battery, and triple rear cameras. We reviewed the phone when it was first released, and while it does offer a clean Android experience and speedy performance, image processing isn't the best and the phone is a bit slippery. You'll probably want to pick up a case.
The phone is on sale for $598, but you may have to scroll down the product page to see the listing by Amazon itself — a third-party listing for $100 more is the default option, at least for me. Amazon is also slightly behind on orders, as the phone won't ship for at least a week (as of when this article went live).