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 date by supplying alternatives to the ones I've purchased. Some of the monitors I recommended were $330 when I bought them but are now $500-700, although sometimes they come back in stock at $350ish.

I made the switch around 5-6 years ago and still think it was one of the biggest upgrades for general quality of life improvements when using a computer.

The only reason I haven't gone 4k is because using one at 100% scaling at 27" or less isn't really feasible due to how small the text is and using a 36" one to be able to comfortably view it at 100% scaling feels too big for using it in a normal desk environment. Personally I'd rather have the flexibility of 2x 24-25" 2560x1440 monitors, plus 120hz / 144hz 2560x1440 monitors are very abundant if you're into games (although you can make a strong case that 120hz+ is very noticeable and useful for general usage too).