Best monitor to replace my 24" Apple LED Cinema Display?

Apple is known for their excellent IPS monitors, so it’s very hard to come close to Apple color accuracy without buying a high end display. If you can pull it off, look at either an IPS Dell Ultrasharp or HP Z monitor*. In addition to HP and Dell, LG sells the 5K UltraFine display but it only really works well on Macs due to the lack of input controls on the monitor.
*IMPORTANT NOTE: Many of the commercial HP displays use full size DP and may or may not include HDMI, so you may need to get a DP>mDP adapter (TB2/mDP Macs) or a DP>HDMI cable (2016-present rMBP).

The Ultrashrp IPS monitors - especially with factory calibration, tend to cost a bit more due to most of them using LG IPS panels, instead of Samsung. However, the color performance with LG IPS is excellent, but you do pay for it. Samsung is also good, but isn’t as good with gradients as LG (in part due to how Samsung tends to be 8-bit IPS, while LG is usually 10-bit). Try to avoid 8-bit IPS and look for 10-bit, especially if you care more about matching how excellent Apple’s displays look.

Both Dell and HP have solid warranties on their pro grade displays. That said, Dell tends to have the overall better warranty with many of the high end Ultrasharps having a 0 dead pixel 3 year warranty, whereas HP only includes a typical 1 year (3 year requires a warranty upgrade) warranty and doesn’t have the 0 dead pixel policy. No matter which one you get if you can afford it, the sticker shock is worth it.

The Apple monitors look so good due to their use of LG IPS displays. Samsung no longer sells TFT panels and focuses on QLED, but still offer IPS. While Samsung’s IPS panels aren’t as good in many situations it is really good as well when done right, especially 10-bit IPS. You just need to watch for 8-bit panels with poor gradient performance.

If you can’t swallow the Ultrasharp or HP Z premium, look into the ASUS PROART series or even the professional BenQ monitors. None of the cheaper options will come close to matching the color performance Apple is known for, so you may need some aspirin and an open wallet to get similar or superior quality.

Unless you do not need IPS or dead accurate colors and can skimp a bit, you’re usually going to have to buy these monitors online. Most retailers stock what sells and that is FHD TN.