Format: Quicktime

Format Options-Video Codec: Animation-Quality: 85 to 100, depending on how critical quality is. 100 is uncompressed.

Channels: RGB + AlphaDepth: Millions of Colors+ (the "+" indicates the alpha channel)Color: Straight (Unmatted) - This avoids a luminance shift in semi-transparent areas that you get with "Premultiplied (Matted)"

Others use TIFF or PNG image sequences instead of Quicktime Animation, but the underlying concept is the same.