While not strictly a gaming monitor, the LG 27UK850-W is one of the most feature-packed, high quality displays I've seen on sale for Amazon Prime Day. It's a 4K display in a seriously pixel dense size at 27 inches and has an IPS panel for better viewing angles and color quality than a cheaper TN. Unusually for a more professional-focused monitor, it also has a fast 5ms response time and supports FreeSync, which is nice for gaming (it's not officially on Nvidia's list of approved G-Sync compatible displays, but folks online say it works).
There are some other nice features here too, including HDR10 support and a USB Type-C port for video or charging. Great for just keeping your phone gassed up or even plugging in a tablet.
LG 27UK850-W 27-inch 4K FreeSync monitor | 4̶4̶9̶.̶9̶9 $379.99
This professional/gaming model is the rare example of one that can do both, with a fast 5ms response time, HDR support, and even USB-C.
It's a great list of features for the price, though don't expect top-of-the-line performance in every category here. For example, though this monitor supports HDR10, its 400 nits brightness isn't enough for a great HDR experience (for that you'd want at least 1000 nits). And it's stuck at 60Hz, while most of the best gaming monitors we recommend can hit at least 120Hz.
You can read more in-depth about the positives and drawbacks of the monitor at our sister site Tom's Hardware, but the bottom line is it's pretty damn good: "LG has packaged an excellent gaming monitor and a decent professional display into a single product. The 27UK850 successfully bridges two categories, leaving out very little and selling for a reasonable price."