first of all, make the texture a power of two. (1024, 2048, 4096 on each side)
that helps with mipmapping and such.
disable alpha channel.
set mipmap to normal, or sharpen it a bit.
disable SRGB.
Worst case, set it to uncompressed in the compression settings, but that would make the filesize rather big.
other tips: use a bigger font, grayscale the texture.