Congrats on the new MBP! I almost pulled the trigger on a new one when they came out, but then I realized that with my 2016 13” touchbar version not really requiring an upgrade, I was falling into the distortion field…Still, that quad-core I7 or even i9 with 32GB is looking tempting…
Anyway, I would think that virtually any modern monitor with the appropriate connectors would function, so the decision is really one of cost and features that you need.
I presently have two monitors hooked up. I have a Dell 24” 4K (the “Katie Floyd” model) which I recently got (and selected based on KF’s comments on it) which I am extremely happy with. I was previously using an LG 34” widescreen (the one with Thunderbolt, which was on the market for a short period of time, very expensive (I got a huge discount so it was worth it at the time). I found that in contrast to others who work well with a huge monitor, this one was too large and wound up being distracting more than a benefit. I have it hooked up as a secondary monitor and have yet to move a window on to it, so as soon as I find a buyer, it’s moving to a new home. I will likely get a second 24” Dell (not 4K) and place it vertically as David has recently done.
I think you should first determine how much you can afford to spend, as that is the basic factor you need to establish your real of possibilities.
Next, think about size. I have found that working on a smaller monitor with focus on the app I am using has helped me vs having a jillion windows open with things distracting me from the work at hand. With the use of Spaces and swiping, this works well for me. I keep one Space for the current app with everything else behind it; one space just for Mail maximized (option—click the green button, so it is not a full screen space, just a maximized app in a regular space) so I can quickly swipe over to it, and a third space that is empty, which I jump over to when I need to quickly get an empty screen (I often use that when I need to open a couple of Finder windows and move things around quickly).
Next up: what features are important to you. For example, 4K is really very nice; I think the higher resolution makes everything sharper and easier to read and I find an easier transition from the retina screen when the MBP is used disconnected from the monitor. Are you a gamer? Then you want a monitor with the highest refresh rate. Photographer? Get something with accurate color and the best color space coverage (the 24” I have is very good in that regard and I am a photographer, so…I could go the very expensive route of multi thousand dollar monitors designed for this purposes, but the Dell is good enough that the cost difference was not worthwhile for me). Is having built in speakers or a USB-3 hub crucial for you, or can you work with external speakers (I put a Dell USB soiund bar on the monitor which is fine for most things, and I can always pipe music through my Sonos so I don’t need ultra high quality speakers built in to a monitor).
Finally, I happen to think it’s really worthwhile to try to see the monitor in action OR buy from a place like Amazon or BHPhoto with a liberal return policy. I was looking at an LG 27” 4K for a while, but when I looked at it at Best Buy, I was surprised at how dim it appeared compared to adjacent monitors and I did not like the image. I played with the settings, found it was already cranked to the highest brightness, and found I just didn’t think the image worked for me. Obviously this is very personal. I bought the Dell sight unseen as it was not available anywhere locally, but bought from Amazon in case I hated it. As it turned out, I really liked it but if I had not, at least I could have sent it back.
I hope my rambling is useful.