A 2560x1440 monitor running at its 1:1 native resolution. The screen real estate improvement over 1080p is substantial. You get so much more vertical space and you can easily fit 4 side by side code windows at 80 chars. At 24" or 25" the PPI is also quite nice for reading text. I wrote up a very big monitor selection guide at https://nickjanetakis.com/blog/how-to-pick-a-good-monitor-fo... , I try to keep it up to dat…
MacOS in particular does an excellent job of rendering 2560x1440 to a 4K screen, and the increased DPI over a regular 1440p 27" screen is very noticeable.
Another option if you're not a fan of 27" displays is a pair of 24/25" 4k screens which can be run at a scaled 2304x1296 resolution. This still provides a decent amount of space without text being too tiny. Alas, 4k monitors <27" are increasingly rare these days.