The first thing to consider for pretty much any purchase is budget because this will decide a lot of other factors for you. If you need to keep prices low, you'll need to consider a lower screen resolution than you might if money is no object.
If you simply must have a high-resolution screen for very detailed image work, for example, then you'll have to spend a bit more. But again, budget comes into play about which monitor you'll choose: some 4K displays (or even higher resolution) have high-end additional features and greater brightness, but you can get a monitor that focuses more on resolution and colour accuracy to give you just what you need for less (like our second place choice).
If you spend a lot of time connecting and disconnecting your MacBook Pro to take elsewhere, you should look at USB-C monitors, because these can connect to the laptop over a single cable carrying power and the video stream, making it extra easy to plug and unplug for hitting the road.
In all cases, we recommend looking for monitors with adjustable height at least, so that you can make sure it's good to work at ergonomically. In a world where more and more people are working from home, this is one area you don't want to skimp on when you buy, back pain isn't funny and having to stoop or stretch for long periods isn't going to help your health.