Compressed color with alpha channel texture format.
DXT5 (BC3) format compresses textures to 8 bits per pixel, and is widely supported on PC and console platforms.
It is a good format to compress most of RGBA textures. For RGB (without alpha) textures, DXT1 is better. When targeting DX11-class hardware (modern PC, PS4, XboxOne), using BC7 might be useful, since compression quality is often better.
Additional resources: Texture2D.format, texture assets.