Standards change all the time, but the bare minimum right now is support for HDR and Dolby Vision, and at least HDMI version 2.0 or better. All of these models support not only 4K and HDR video but 8K support as well, even if 8K content is hard to find.  Be aware that all 2020 8K-compatible receivers were prone to a bug preventing them from displaying variable refresh rate video, and from the Xbox Series X in particular. Denon, Marantz and Yamaha announced fixes for existing models, while compliant models from Yamaha RX-V6A began shipping in summer 2021 and Denon and Marantz receivers sold after April 2021 should be 4K/120Hz compatible. Yamaha users can check for 4K compatibility here while Denon and Marantz users should check with their dealer. The $800 TX-NR6100 is the first receiver I tested that I found to both pass 4K/120Hz and which I would also recommend to new buyers.