The most common CRS for online maps, used by almost all free and commercial tile providers. Uses Spherical Mercator projection. Set in by default in Map's crs option.
A common CRS among GIS enthusiasts. Uses simple Equirectangular projection.
A simple CRS that maps longitude and latitude into x and y directly. May be used for maps of flat surfaces (e.g. game maps). Note that the y axis should still be inverted (going from bottom to top).
Rarely used by some commercial tile providers. Uses Elliptical Mercator projection.