High Dynamic Range is without a doubt a much bigger deal than 8K, and certainly something you should consider before buying your next TV.
Essentially, the higher the dynamic range (brightness and colours), the more lifelike the picture. HDR offers greater subtlety and depth of gradations of colours, plus stronger contrast.
There are various types of HDR out there, and with different TV brands backing different variants, it can be a minefield trying to find the best option. Allow us to explain.
First up is HDR10, which is essentially the core HDR format that every HDR TV should support.
HDR10 is a static HDR format that applies the HDR values on a scene-by-scene basis (i.e. whenever the camera cuts to a new scene). Dolby Vision, on the other hand, applies this image information (called metadata) on a frame-by-frame basis. This dynamic form of HDR, when implemented properly, has the potential to improve upon the standard HDR10 presentation.
HDR10+ is a rival format to Dolby Vision. Created by Samsung, it also uses dynamic metadata but, whereas Dolby Vision is licensed, HDR10+ is a free, open format that any company can deploy as it sees fit.
Of these two 'dynamic' HDR formats, Dolby Vision is by far the most prevalent, both in terms of TVs and content, and if you have to choose between one and the other, that's the one we'd recommend. That said, you can now buy TVs from the likes of Philips and Panasonic that support both Dolby Vision and HDR10+.
Finally in our rundown of HDR formats is HLG (Hybrid Log Gamma), which was developed specifically for broadcasting by the BBC and Japan's NHK. It's used to deliver all of the HDR content offered by the BBC and Sky, so can be considered very important. Luckily, it's now almost as common as standard HDR10 in TV spec lists, so you should have little problem finding a model that supports it.