ProPhoto RGB
ProPhoto RGB is a very large color space that holds more colors than any screen or printer can show. Lightroom uses it internally to preserve all your RAW color information while editing. So you edit in it, but you almost never export in it.
The advantage of ProPhoto when editing is that you lose no color, however far you push the sliders. The downside: export a JPEG in ProPhoto and colors look wrong everywhere, because no device understands that space. Keep it simple: edit in ProPhoto (Lightroom does this automatically) and export to sRGB for web or Adobe RGB for print. Working in 16-bit gets the most out of such a large space without color banding.
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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.