Casey Liss has compiled a list of viable Mac monitors, and I think it's a decent list, but I'd like to add to the discussion since I think his standards are a bit too specific.

I'm using the LG 27” IPS 4K UHD Monitor (model #: 27UP600-W.AUS) which cost me about $399 and technically falls outside of Casey's criteria, but I think works wonderfully. It's 4K, supports the P3 color space, and has the inputs I need for my (realtively) basic needs.

It also happens to look a lot better, in my opinion, than the budget option in the article.

Now there are a couple downsides, of course.

1. HDR 400 is basically not HDR at all, so don't buy it for that.
2. There's no Thunderbolt/USB-C connector, so no daisy-chaining things together.
3. The interface is a bit clunky, but that's only an issue if you switch inputs (it does not auto-detect if you switch inputs, like some monitors helpfully do).
4. To display at native 2x, you have to output at 1920x1080, which is a bit cramped depending on your use case. It looks unbelievably good to my eyes, though.

So that's it, that's my recommendation for a 4K monitor that works well with the Mac, looks nice enough, and is at a reasonable price point. I would love to see Apple put out something in the sub-$2,000 range, but I'm really not sure what that would look like at this point.