I’m close to throwing in the towel on the Target Display Mode thing.  Dead end after dead end.  Works, then stops and goes to a black screen.  
I have the new black 27” iMac Pro.  Do you have a recommendation for what the top of the line monitor would be that I can connect to it?
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Glad to hear you finally came to your senses. I’m sure you fought the good fight. I think it’s disgusting that Apple didn’t build that very simple option to just continue to use the iMac as a monitor other than that small window of time plus the fact that you have to actually have the computer booted into a user. Stupid. That defeats the purpose. They could have easily built in a key combo that you hold down when booting that allows it to just act as a pure monitor. So if you feel like filing a class action law suit on them, count me in as a witness.

So second issue. Monitors are like ice cream. You may be totally happy with Food Lion brand vanilla or your could spend 3 times more to get Bryers.

Things to look out for when purchasing a monitor:

1. What resolution will the actually machine you’re using push onto a second monitor? I suspect with the iMac Pro, you can go bigx2.
2. Resolution
3. Refresh rate
4. Contrast ratio
5. Resolution!

Buying a monitor that is a different resolution than the main iMac can be a little weird. Totally doable but moving a window from one screen to the other to see its size change is a little low budget. I prefer to have the exact resolution as my main. Not all monitors are equal. Just because it says it supports a certain number doesn’t mean it will look good at that resolution. Don’t buy a weird size like some super wide curved display. Total cheese.

Brands I like:
LG
Samsung
Dell
Toshiba

Brands to stay away from unless money is tight:
Spectre
Viewsonic
Acer

I owner several Spectres because their cheap. The contrast ratio sucks which means black is not really very dark and white is not all that bright. But for my use of them just being side monitors for email or a calendar, that doesn’t really matter. I wouldn’t edit photographs on it.

There’s a very good chance your iMac monitor will be far superior to your second monitor. Apple has very good monitors and most monitors under $350 will not compare. That said, I have a 32” LG monitor I just bought that I use in tandem with a 16” MBP and I’m really high on it. I will probably end up buying another LG 32 within the next 6 months.

Industry secret: Every monitor contains dead pixels. You’ll most likely never notice them however because they are usually just dead sub pixels. Each pixel is made up of 3 sub pixels: Red, Blue, Green. A very expensive montior is going to prevent light from coming through the closed pixels therefore giving the appearance of a true black. All black is on a monitor is all three pixels closed. If the monitor is a poorer quality, there will be light that leaks through and the black will be less black. You see the same concept on cheap TV screens. Some tv’s are now able to turn off the back light behind pixels in different areas of the screen but those are $$$.

Anyway, long story short: LG, Samsung, High Contrast ratio, resolution that matches your primary. But be ready to $$$ to get close to the iMac resolution.