Rec.709
Rec.709 is the standard color space for regular HD video, as shown on most screens and on the web. It defines which colors and what contrast count as normal. If you deliver for YouTube or TV, Rec.709 is almost always your final destination.
If you shoot in log or RAW, you eventually convert your image to Rec.709 so it looks right everywhere. In DaVinci Resolve, Premiere Pro and Final Cut Pro you set this via your project settings or a color space transform. Grade in the wrong space and colors will look very different on the viewer's screen at home. Tip: check your grade on a Rec.709 monitor, because that's what your audience will see too.
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The presets on this site set these adjustments up for you as a starting point, which you then fine-tune to taste.