There is no "ideal non-Apple display". It depends on what you're looking for in a monitor, what you do with it, and your budget.
For a 27" monitor, people generally recommend a 1440p monitor (non-retina) or a 5K monitor (retina / double the resolution of 1440p). This is because of how scaling works on a Mac. For this monitor size/resolution, macOS will default to what "looks like 1080p". Things will be sharp and crisp, but the UI will be large. If you switch to 4K, then things will be tiny. You can make it "look like 1440p" to have a comfortable sized UI, but that
could affect performance depending on what you're doing with the Mac (
example).
Also, things won't be pixel perfect when using "looks like 1440p" on a 4K monitor because of the non-integer scaling it has to do (2160p isn't divisible by 1440p). It may be fine for office/everyday type stuff, but not if you need to do pixel perfect photo/graphic editing. For example, say you have a line 1 pixel wide...when using "looks like 1080p", this line is 2 pixels wide; at 4K this line is 1 pixel wide; at "looks like 1440p", this line needs to be 1.5 pixels wide. Obviously you can't have fractions of a pixel, so it'll show up as 1 or 2 pixels wide.
That said, many people do use 27" 4K monitors just fine and you generally go wrong with Dell UltraSharps. It really depends on your situation.