ISO images are most well known and used.. For that id say they are the most popular.. The best, well, hard to say as there aint that many competitors with large usage.. Most popular images can be mounted as a loop device, so you can fool it as a cdrom if you must, or just use it as a regular folder.. heres an example for a iso image:
su -c "mount -t udf,iso9660 -o loop,ro,nodev,noexec,nosuid /path/to/image.iso /mnt/cdrom"
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