Theory is that a video will be sharpest on screen that's the same resolution as the video, or an exact multiple.
In practice the way the encoding works means there's little or no difference, especially with low res videos - a 720p vid will look just as good on a 1080p or 4K screen as it does on a native 720 or 1440 screen.
Rather oddly, in the past I've found the main differentiating factor in video quality is the actual LCD panel itself; I once owned a pair of Yuraku 24" monitors, both the same model and identical except one used a Samusng S-PVA panel and one an AUO MVA panel. With both monitors hooked to the same PC in clone mode the MVA one always produced better quality video, with less pronounced compression artefacts. The difference wasn't huge, but was enough to be noticeable from a few feet away.