What is the best 24" monitor for Mac mini?

Not much available in 24" these days, but here are two for you to consider:

LG 24UD58-B (24", 4K, IPS)

• It has an IPS panel, decent color accuracy out of the box, and is generally well-reviewed for macOS clarity. At 24", 4K gives you ~183 PPI, which is just shy of Retina density.
• It doesn’t have USB-C, so you’ll use HDMI or a Thunderbolt to DisplayPort adapter, but it works cleanly with Mac minis.

Dell UltraSharp U2520D (25", QHD, USB-C)

• Slightly larger than you asked, but still compact. It's 2560x1440 resolution, so not true Retina, but its panel quality, brightness, and factory calibration make it great for macOS scaling.
• It has USB-C, so with a USB-C to USB-C cable, it can power and display from a dock.

If sharp text is your number one priority, go for 4K at 24", even if it narrows your choices. Also, make sure to adjust macOS display scaling to “Looks like 1920x1080” for Retina-style rendering — macOS will render at 3840x2160 and downscale, which helps smooths text.

Personally, I have been using Dell monitors, with my Mac minis, for years and have been happy with them. Regardless of which brand you go with, check out the seller's return policy in case it doesn't work out for you.

I've had much success with LG monitors they seem to also make the LCD panel themselves for a lot of other monitor makers.