Glad that it helped, thanks for letting us know!
anaskhan If i override it then which compression is best ASTC or ETC2?
Any format has its advantages and drawbacks.
ASTC is a more recent format which also offers non-standard block size (is not restricted to blocks of 4x4), and thus could yield better results with arbitrary texture sizes. It's faster to compress, which is not too much of an issue during development and completely irrelevant at runtime. On the downside, it may not be supported on all older devices. Then however, Unity will decompress the textures at runtime (as documented here) and then use an uncompressed 32bit format.
Quality-wise, it's always best to run some side-by-side comparison, or even better, exchange it at the exact same location, on your target devices. Also, be sure to try with different settings, and note that Compressor Quality determines the time in the Editor that the compressor takes to optimize the output image quality, not runtime speed. Setting it to Fast (instead of Normal or Best) makes your import faster upon frequent atlas texture changes, but does not affect runtime performance, it does not make the game run faster.
