What is the best monitor for the Mac mini

Best is in the eye of the beholder. What looks good to me or someone else, may not be what looks best to you.

Small is a matter of perspective. These days a 21.5" monitor is considered small when compared next to a 27" or 32" monitor.

Your best bet if possible, is to visit your local Electronics Retailer (BestBuy, Staples, OfficeMax, etc..) and compare some different brands and sizes side by side.

Most all new computer monitors can be connected to the Mac Mini.

see > https://support.apple.com/guide/mac-mini/connect-a-display-apd8e4fbbb97/mac

YMMV. I have LG UltraFine 4K 23.7" Thunderbolt display and it is nice with Mac mini 2018.

Pros: minimal cable clutter, keyboard shortcuts to audio loudness and video brightness, IMHO good image quality with Night Shift, not too small letters with some eye-sight (I wear +1 glasses with it but can also do without).

Cons: A bit pricey, the stand can not be lowered (tilting OK), only thunderbolt.

p.s. during the very first months Mojave had back screen at Recovery mode which lost me some hair but it was fixed with a Mojave update. Firewire devices caused random few second blackouts but a new USB-C HD dock fixed that.

I recently purchased the Dell U4021QW 40" curved wide screen 5120 x 2160 monitor connected via USB-C Thunderbolt4 cable on a brand new M1 Mac mini. The resolution is about 30% wider than a 4K monitor. Vertically it's 4K but the width is more than 4K. Text appears sharp and high quality. It is 60Hz on the refresh rate. I am very happy with it. I still wish Apple would create some new monitors because I would buy them. I cannot justify the cost of the XDR monitor that is just insanity.

It has ports including Ethernet and acts as a kind of dock and yes the Ethernet works but I don't need it for the Mini but it works fine for the other computers I have connected. You can connect more than one computer to it and it even works with a Nintendo Switch (albeit with black bars on the side). Works well with my work 15" MacBook Pro 2019 as well an an HP Elitebook X360 1030 G7 Win10 laptop. It has built-in speakers but controlling them is a bit flakey with the Mac Mini. I ended up using external speakers connected to the Mac. The Nintendo Switch uses the built-in speakers via HDMI just fine. Due to the odd resolution there are not many wallpapers that will fit it properly. I've resorted to making my own wallpapers by editing dual screen wallpapers merged together to fit.

The only downside is it doesn't work with the display brightness buttons on the keyboard. Software workarounds that could use DDC/CI do not work on the Apple Silicon based macOS Big Sur because Apple apparently forgot to implement DDC/CI in their kernel like they did on the Intel kernel. Might have to do with some specific Intel requirements that no longer apply with Apple Silicon. I found an App called QuickShade that will dim the display but I don't think it's actually dimming the display in the same way. It works but it has a bug with screenshots where I cannot select a window. I have to quit the QuickShade app to take a window screenshot. I think it's created a shaded transparent window on top of everything that is appearing to dim the display without actually dimming the display. You can still adjust the brightness manually with the monitor controls.

There’s a bewildering number of display available with different sizes, resolutions, ports, picture qualities, special features and price points that make it almost impossible to recommend a best monitor. Making it even more difficult is that most manufacturers have lots of different offerings ranging from amazing to mediocre. My solution has been to visit the big box stores and look at what they have as well as checking out what my friends are using. That’s how I found the last two displays I’ve purchased.

That said, I echo Rudegar’s recommendation for either HDMI or DisplayPort. My current display (from LG) has two of each port which makes it easy for me to use it with both my mini and notebook. Don’t look just at size, look at resolution too; there are low resolution displays with large LCDs. Finally, one way that manufacturers save a bit of money is to provide a truly lousy stand. The display I currently have is everything I wanted - but it shook every time I moved my mouse. Buying a third party stand solved it.

one that support 1080p and use hdmi or displayport

The best monitor for the Mini would be the Apple Pro Display XDR but it’s $6000.00. 😎

I use a Thunderbolt display (although out of production) and works great. It has the speakers, camera and ports.